Add virtual manifest support

[bjgill]

In a workspace with a virtual manifest (one with [workspace], but not [package]), we get the following error message:

$ cargo tree
error: manifest path `<path>/Cargo.toml` is a virtual manifest, but this command requires running against an actual package in this workspace

I suspect it should be fairly straightforward to switch over to using https://crates.io/crates/cargo_metadata to get the set of dependencies and their relationships. Cargo-metadata is fully workspace-aware - we use it in cargo-edit for this purpose

[sfackler]

What would the dependency graph of a virtual manifest be?

cargo-metadata doesn't include enough information for us to be able to use it: #20

[mimoo]

@sfackler it would be the result of running cargo tree on all the listed members.

[mlindner]

@sfackler Please re-open this. Cargo tree needs to support virtual manifests. It should iterate through all entries in the virtual manifest. Cargo.toml lists a list of subpackages and it should show the trees for all of them.