Let's be always explicit whether we create a library (i.e., an immutable
dependency) or a local `SourceRoot`, since it can have a large impact on
the validation performance in salsa. (we found it the hard way recently,
where the `Default` instance made it quite tricky to spot a bug)
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>
