Respect package boundaries in bp2build conversion of go modules

bp2build's codegen context does not implement
BazelPathConversionContext. To reuse the utility function
transformPackagePaths, update its signature

(Also make deps of go_library unique to make the conversion resilient)

Test: go test ./bp2build
Change-Id: I126b1057d2b26bc6c7d3be2780f1b62d28323cf0