`ResPath.RelativeTo()` now considers non-rooted paths relative to `.`
This fixes some things like `MemoryContentRoot`'s `FindFiles()` implementation.
Fix `IContentRoot.GetEntries()` default implementation (used by all content roots except `DirLoader`) not working at all.
Made `ResourceManager.ContentGetDirectoryEntries()` report content root mount paths as directories.
Added tests for all of the above.