bp2build: add bazel_module: { bp2build_available } prop.

This CL adds a per-target allowlist to instruct bp2build on which modules it should generate Bazel targets for.

Test: soong tests
Change-Id: I869e66fce405c2c6689b381569b8cc0118cbcf76
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index 2c293ea..2c553c6 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -213,6 +213,8 @@
 	// Delete it from being generated in the BUILD file.
 	delete(props.Attrs, "bzl_load_location")
 
+	delete(props.Attrs, "bp2build_available")
+
 	// Return the Bazel target with rule class and attributes, ready to be
 	// code-generated.
 	attributes := propsToAttributes(props.Attrs)