997: Improve filtering of file roots r=matklad a=vipentti
`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.
Relates to discussion in #869
Co-authored-by: Ville Penttinen <villem.penttinen@gmail.com>
b038832795