The hcl2template package contains references already, but these are
linked to a particular type.
This becomes problematic if we want to support cross-type references, so
this commit adds a new abstraction: refString.

A refString contains the component type, its type (if applicable), and
its name, so that the combination of those points to a cty object that
can be linked to a block in the configuration.

Right now, only `var`, `local` and `data` are supported, but the type is
extensible enough that anything else that fits this model such as
sources can be supported in the future potentially.

The `Name` function is used by the DAG library to determine which vertex
is associated to a component, so the `Name` attribute/function needs to
replicate the combination of type and name, so the vertex can be
accurately fetched from the graph.

Since we are in the process of integrating a new way to orchestrate
dependency management and evaluation for datsources and local variables,
we need to split the current function that manages the evaluation of
said local variables, so that recursion and the actual evaluation of a
local variable are two separate functions.

This will allow for evaluating a single variable once the dag is ready
to be introduced.

As with variables, this commit introduces a new function whose purpose
is evaluating the datasource directly, without looking at its
dependencies, or recursively trying to execute them.

Instead, we rely on the DAG to determine when it is safe to execute it,
or if using the phased approach, the current logic will apply.

When registering dependencies for datasources and locals, we now use
refString.

This allows for the functions that detect dependencies to not only be
able to register the same types as dependencies, but instead generalises
it to any type that refString supports, so data, local and var.

This can then be leveraged for orchestrating evaluation of those
components in a non-phased way (i.e. with a DAG for dependency
management).

The dag package is a port over from Terraform to Packer, changing what
little there was to fit our current dependency ecosystem.
Most of the changes are on the type of diagnostics returned, as
Terraform has its own type for them, while we rely on hcl's Diagnostics.

Other than that, the functionality is essentially equivalent, and the
code was barely touched.

As we have finished setting-up the codebase for it, this commit adds the
logic that uses the internal DAG package, and is able to orchestrate
evaluation of datasources and locals in a non-phased way.

Instead, this code acts by first detecting the dependencies for those
components, builds a graph from them, with edges representing the
dependency links between them, and finally walking on the graph
breadth-first to evaluate those components.

This can act as a drop-in replacement for the current phased logic, but
both should be supported until we are confident that the approach works,
and that there are little to no bugs left to squash.

Following up on the DAG work, this commit adds a new option for
initialisation that disables DAG on request.

By default we are going to use the DAG approach, with an option to
fallback to using the older algorithm for evaluation in case users
end-up in an edge-case that prevents them from building a template.

For all the commands that call Initialise, we introduce a new flag:
UseSequential.

This disables DAG scheduling for evaluating datasources and locals as a
fallback to the newly introduced DAG scheduling approach.

`hcl2_upgrade` is a special case here, as the template is always JSON,
there cannot be any datasource, so the DAG in this case becomes
meaningless, and is not integrated in this code path.

The implementation of the DAG as extracted from Terraform relied on a
Root vertex being injected into the graph as the last node to visit.

This is used as a sanity check for Terraform, but doesn't apply to our
use-case for now, as we are always executing everything and have no need
for this root node.

Instead, we change how Validate operates so it does not error in case
there is no valid root node for the graph, but enables us calling it to
check for self-referencing edges, and circular dependencies.

Local variables had an attribute called Name with the name of the local
variable.

However, when producing an error while walking the DAG of
local/datasources, if an error is encountered during validation, the raw
structure of the vertex was printed out, making the error message
produced hard to understand.

Therefore in order to clean it up, we rename the `Name` attribute for
Local variables as `LocalName`, and introduce a `Name()` function for
that block so that the complete name of the variable is clearly
reported.

Walk uses a reverse topological order to walk on the graph, doing that
visit concurrently if possible.

This is nice as we can speed-up execution of datasources and locals,
however since the `Variables` map stored in the config, and the
production of the context for it, are not meant to be used concurrently,
this means that we end-up in cases where Packer crashes because of
concurrent accesses to that map.

So until we can change this behaviour, we will fallback to using the
sequential visit algorithm for those vertexes, therefore limiting the
risk of those conflicts.

When preparing the datasources to add into the DAG for evaluating the
build prerequisites, we ended-up in a weird situation in which the
datasources for each vertex pointed to the same one.

This is because of the loop semantics of Go, where the same object is
reused over and over again during each loop, so in the end every
datasource vertex pointed to the same instance of a datasource block.

To avoid this, we instead grab them through their reference, making the
reference to the datasource purely local, and pointing to the actual
datasource block, not the one scoped to the function.

Evaluating local variables used to be directly written to the
PackerConfig while each variable was created.

This was somewhat of an issue with testing, as we have a bunch of tests
that relied on `PackerConfig.Variables` being set only when we actually
write something.

This is not really a concern for normal use, just for testing, but to
limit the number of changes to the tests in hcl2template, I opted to
change how variables' values are retained, so that evaluating a single
variable returns a Variable in addition to hcl.Diagnostics, so we can
reify the approach and only create the map of variables if there's
something evaluated.

Since we introduce the DAG with this series of commits, only on locals
and data sources, we need to make sure that the behaviour is what we
expect.

Therefore, this commit adds a basic test with Packer build, and packer
validate, to evaluate a template with both locals and data sources
depending on one another.

This is rejected with the sequential evaluation methods, as we process
the different types one-by-one, whereas the DAG allows us to mix the
order between the two, while still rejecting circular dependencies (and
doing that before they even get evaluated), and self-references.

Since the DAG package was lifted from Terraform, its contents are more
than what we need for now, so this commit cleans-up the package to keep
only the currently needed parts of code.
If we need to support more in the future, we can revert this commit, or
pickup the changes again from Terraform.