create, but dont register, bp2build_deps mutator

This is the bulk of the "allowlist v2" feature. It will disable bp2build
generation for modules which have transitive dependencies without a
bazel build definition.

This CL includes this mutator, but doesn't register it as a bp2build
mutator (outside of a few unit tests). This allows us to easily iterate
on completion of this feature and ensure there are no launch blockers
before we finalize the change in AOSP.

Bug: 285631638
Test: Unit tests
Change-Id: Ifb0a079c409ca19b02cafa3fab2efa0d3deebc50

android/config.go

@@ -297,6 +297,10 @@ type config struct {
 	// in tests when a path doesn't exist.
 	TestAllowNonExistentPaths bool
 
+	// If testAllowNonExistentPaths is true then PathForSource and PathForModuleSrc won't error
+	// in tests when a path doesn't exist.
+	Bp2buildDepsMutator bool
+
 	// The list of files that when changed, must invalidate soong_build to
 	// regenerate build.ninja.
 	ninjaFileDepsSet sync.Map