Improve filtering of file roots

`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.

crates/ra_lsp_server/src/lib.rs

@@ -4,6 +4,7 @@ mod conv;
 mod main_loop;
 mod markdown;
 mod project_model;
+mod vfs_filter;
 pub mod req;
 pub mod init;
 mod server_world;