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/bp2build/conversion.go b/bp2build/conversion.go
index 7877bb8..787222d 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -21,14 +21,15 @@
 	mode CodegenMode) []BazelFile {
 	files := make([]BazelFile, 0, len(ruleShims)+len(buildToTargets)+numAdditionalFiles)
 
-	// Write top level files: WORKSPACE and BUILD. These files are empty.
+	// Write top level files: WORKSPACE. These files are empty.
 	files = append(files, newFile("", "WORKSPACE", ""))
-	// Used to denote that the top level directory is a package.
-	files = append(files, newFile("", GeneratedBuildFileName, ""))
-
-	files = append(files, newFile(bazelRulesSubDir, GeneratedBuildFileName, ""))
 
 	if mode == QueryView {
+		// Used to denote that the top level directory is a package.
+		files = append(files, newFile("", GeneratedBuildFileName, ""))
+
+		files = append(files, newFile(bazelRulesSubDir, GeneratedBuildFileName, ""))
+
 		// These files are only used for queryview.
 		files = append(files, newFile(bazelRulesSubDir, "providers.bzl", providersBzl))