bp2build: split as, c, and cpp srcs for cc_library

This CL adds support for cc_library to correctly split c, as and cpp
srcs in shared/static nested props, as well as splitting
the *filegroup* deps in those props, where each filegroup is expanded
into its own c, cpp and as srcs filegroups. This ensures that the
correct sources go into cc_library_static's underlying cc_libraries for
c, cpp and as sources respectively.

See the bp2build conversion test for a better visualization.

Bug: 183064430

Test: TH
Change-Id: I29add5140672d042adff65527d8b65f4a5f0a05b
diff --git a/android/filegroup.go b/android/filegroup.go
index fc6850e..e207412 100644
--- a/android/filegroup.go
+++ b/android/filegroup.go
@@ -63,7 +63,10 @@
 		Srcs: srcs,
 	}
 
-	props := bazel.BazelTargetModuleProperties{Rule_class: "filegroup"}
+	props := bazel.BazelTargetModuleProperties{
+		Rule_class:        "filegroup",
+		Bzl_load_location: "//build/bazel/rules:filegroup.bzl",
+	}
 
 	ctx.CreateBazelTargetModule(BazelFileGroupFactory, fg.Name(), props, attrs)
 }