`ProjectWorkspace::to_roots` now returns a new `ProjectRoot` which contains
information regarding whether or not the given path is part of the current
workspace or an external dependency. This information can then be used in
`ra_batch` and `ra_lsp_server` to implement more advanced filtering. This allows
us to filter some unnecessary folders from external dependencies such as tests,
examples and benches.
Instead of using canonicalize, we now join the given path to
`std::env::current_dir()`, which either replaces the path, if the given path is
absolute, or joins the paths.
This fixes#821.
