If the package is know, we return the corresponding file, whether it
exists or not.
If the package is not know, there is no valid Source to resolve to,
so we return null.
We don't need all the features that existed.
Internally we switched to BazelPackageUriResolver, which has its own
logica for searching files in workspaces across multiple folders.
All the places where we create PackageMapUriResolver in analyzer
or analysis_server, we create it with exactly one folder.
R=brianwilkerson@google.com
BUG=
Review-Url: https://codereview.chromium.org/2812093002 .
Konstantin Shcheglov
6fe8e2278b