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/bzl_conversion_test.go b/bp2build/bzl_conversion_test.go
index f2a4058..30c1a5b 100644
--- a/bp2build/bzl_conversion_test.go
+++ b/bp2build/bzl_conversion_test.go
@@ -86,6 +86,10 @@
         "soong_module_name": attr.string(mandatory = True),
         "soong_module_variant": attr.string(),
         "soong_module_deps": attr.label_list(providers = [SoongModuleInfo]),
+        # bazel_module start
+#         "label": attr.string(),
+#         "bp2build_available": attr.bool(),
+        # bazel_module end
         "bool_prop": attr.bool(),
         "bool_ptr_prop": attr.bool(),
         "int64_ptr_prop": attr.int(),