bp2build: framework for generating BazelTargetModules.

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

android/testing.go

@@ -57,6 +57,7 @@ func NewTestArchContext(config Config) *TestContext {
 type TestContext struct {
 	*Context
 	preArch, preDeps, postDeps, finalDeps []RegisterMutatorFunc
+	bp2buildMutators                      []RegisterMutatorFunc
 	NameResolver                          *NameResolver
 }
 
@@ -81,12 +82,27 @@ func (ctx *TestContext) FinalDepsMutators(f RegisterMutatorFunc) {
 	ctx.finalDeps = append(ctx.finalDeps, f)
 }
 
+// RegisterBp2BuildMutator registers a BazelTargetModule mutator for converting a module
+// type to the equivalent Bazel target.
+func (ctx *TestContext) RegisterBp2BuildMutator(moduleType string, m func(TopDownMutatorContext)) {
+	mutatorName := moduleType + "_bp2build"
+	f := func(ctx RegisterMutatorsContext) {
+		ctx.TopDown(mutatorName, m)
+	}
+	bp2buildMutators = append(bp2buildMutators, f)
+}
+
 func (ctx *TestContext) Register() {
 	registerMutators(ctx.Context.Context, ctx.preArch, ctx.preDeps, ctx.postDeps, ctx.finalDeps)
 
 	ctx.RegisterSingletonType("env", EnvSingleton)
 }
 
+// RegisterForBazelConversion prepares a test context for bp2build conversion.
+func (ctx *TestContext) RegisterForBazelConversion() {
+	RegisterMutatorsForBazelConversion(ctx.Context.Context, bp2buildMutators)
+}
+
 func (ctx *TestContext) ParseFileList(rootDir string, filePaths []string) (deps []string, errs []error) {
 	// This function adapts the old style ParseFileList calls that are spread throughout the tests
 	// to the new style that takes a config.