bp2build: add allowlist for package-level conversions.

This CL adds the support for specifying lists of directories in
build/soong/android/bazel.go, which are then written into
out/soong/bp2build/MANIFEST. Using this configuration,
modules/directories can either default to bp2build_available: true or
false, while still retaining the ability to opt-in or out at the module level.

It also ensures that ConvertWithBp2Build returns true iff the module
type has a registered bp2build converter.

Test: go tests
Test: demo.sh full
Test: TreeHugger presubmits for bp2build and mixed builds.

Change-Id: I0e0f6f4b1b2ec045f2f1c338f7084defc5d23a55
diff --git a/android/mutator.go b/android/mutator.go
index 9552aa1..9e99bee 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -226,7 +226,7 @@
 
 var bp2buildPreArchMutators = []RegisterMutatorFunc{}
 var bp2buildDepsMutators = []RegisterMutatorFunc{}
-var bp2buildMutators = []RegisterMutatorFunc{}
+var bp2buildMutators = map[string]RegisterMutatorFunc{}
 
 // RegisterBp2BuildMutator registers specially crafted mutators for
 // converting Blueprint/Android modules into special modules that can
@@ -237,7 +237,7 @@
 	f := func(ctx RegisterMutatorsContext) {
 		ctx.TopDown(moduleType, m)
 	}
-	bp2buildMutators = append(bp2buildMutators, f)
+	bp2buildMutators[moduleType] = f
 }
 
 // PreArchBp2BuildMutators adds mutators to be register for converting Android Blueprint modules