Add deps of protos to bp2build.

Update BazelOutPath to implement genPathProvider. This allows Bazel
outputs (incl filegroup) to be used as an input to generating a file
(e.g. proto).

Test: bp2build.sh
Test: mixed_build.sh
Bug: 200601772
Change-Id: I5ad67ade193025652100b214996b26ce9ca9bf83