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

bp2build/conversion_test.go

@@ -87,18 +87,10 @@ func TestCreateBazelFiles_QueryView_AddsTopLevelFiles(t *testing.T) {
 func TestCreateBazelFiles_Bp2Build_AddsTopLevelFiles(t *testing.T) {
 	files := CreateBazelFiles(map[string]RuleShim{}, map[string]BazelTargets{}, Bp2Build)
 	expectedFilePaths := []filepath{
-		{
-			dir:      "",
-			basename: "BUILD",
-		},
 		{
 			dir:      "",
 			basename: "WORKSPACE",
 		},
-		{
-			dir:      bazelRulesSubDir,
-			basename: "BUILD",
-		},
 	}
 
 	assertFilecountsAreEqual(t, files, expectedFilePaths)