Crate Universe: `crate.spec()` supports `path`

[rickvanprim]

Exposes path on crate.spec() (which just works with cargo metadata 🎉).

However if the path is outside of the Bazel workspace, it attempts to generate dependency paths in the form //<absolute path> (which don't exist), so this splits SourceAnnotation::Path into SourceAnnotation::Path (located somewhere on the file system) and SourceAnnotation::LocalPath (located relative to the Bazel workspace).