This CL creates the framework necessary for generating
BazelTargetModules from regular Soong Android modules.
BazelTargetModules are code-generated into Bazel targets in BUILD files.
See the follow-up CL for examples of creating filegroup/genrule
BazelTargetModules.
Test: GENERATE_BAZEL_FILES=true m nothing # creates out/soong/bp2build
with no BUILD files, because there are no BazelTargetModules in the
module graph.
Change-Id: I33a96365bd439043b13af6db9e439592e9983188