Implement bp2build codegen as a discrete step that runs after an
alternatively registered pipeline of mutators, instead of a
presingleton.
bp2build codegen requires a Context that supports VisitAllModules and
PathContext, so this CL also makes a BpToBuildWrapperContext that
conforms to PathContext by adding two method implementations.
Test: GENERATE_BAZEL_FILES=true m nothing && bazel query //... --config=bp2build | wc -l # 31433
Test: m queryview && bazel query //... --config=queryview # 63638
Change-Id: I0dd359746584b228046d2d0ff00895f28f9bdfc3
daa54bcbba