bp2build: framework for generating BazelTargetModules.

This CL creates the framework necessary for generating
BazelTargetModules from regular Soong Android modules.
BazelTargetModules are code-generated into Bazel targets in BUILD files.

See the follow-up CL for examples of creating filegroup/genrule
BazelTargetModules.

Test: GENERATE_BAZEL_FILES=true m nothing # creates out/soong/bp2build
with no BUILD files, because there are no BazelTargetModules in the
module graph.

Change-Id: I33a96365bd439043b13af6db9e439592e9983188
diff --git a/bazel/properties.go b/bazel/properties.go
index 8bb1956..ac0047b 100644
--- a/bazel/properties.go
+++ b/bazel/properties.go
@@ -19,9 +19,16 @@
 	Label string
 }
 
-// Properties contains common module properties for migration purposes.
+// Properties contains common module properties for Bazel migration purposes.
 type Properties struct {
 	// In USE_BAZEL_ANALYSIS=1 mode, this represents the Bazel target replacing
 	// this Soong module.
 	Bazel_module bazelModuleProperties
 }
+
+// BazelTargetModuleProperties contain properties and metadata used for
+// Blueprint to BUILD file conversion.
+type BazelTargetModuleProperties struct {
+	// The Bazel rule class for this target.
+	Rule_class string
+}