If we get a diagnostic message that references a source range, and if the
source code for the referenced file is available, we'll show a snippet of
the source code with the source range highlighted.

At the moment we have no cache of source code, so in practice this
codepath can never be visited. Callers to format.Diagnostic will be
gradually updated in subsequent commits.

We need to share a single config loader across all callers because that
allows us to maintain the source code cache we'll use for snippets in
error messages.

Nothing calls this yet. Callers will be gradually updated away from Module
and Config in subsequent commits.

As part of some light reorganization of our commands, this new
implementation no longer does validation of variables and will thus avoid
the need to spin up a fully-valid context. Instead, its focus is on
validating the configuration itself, regardless of any variables, state,
etc.

This change anticipates us later adding a -validate-only flag to
"terraform plan" which will then take over the related use-case of
checking if a particular execution of Terraform is valid, _including_ the
state, variables, etc.

Although leaving variables out of validate feels pretty arbitrary today
while all of the variable sources are local anyway, we have plans to
allow per-workspace variables to be stored in the backend in future and
at that point it will no longer be possible to fully validate variables
without accessing the backend. The "terraform plan" command explicitly
requires access to the backend, while "terraform validate" is now
explicitly for local-only validation of a single module.

In a future commit this will be extended to do basic type checking of
the configuration based on provider schemas, etc.

In the long run we'd like to offer machine-readable output for more
commands, but for now we'll just start with a tactical feature in
"terraform validate" since this is useful for automated testing scenarios,
editor integrations, etc, and doesn't include any representations of types
that are expected to have breaking changes in the near future.