Simplify genproto.bzl.

Generate a srcjar artifact, and use native.java_library to emit the
actual jar file.

Post-review modifications:
  - Formatted genproto.bzl
  - Use //external:jar instead of relying on PATH
  - Make proto library a java_import instead of a filegroup in
    test setup.

--
Change-Id: I005f801f09439bbca43e483ad4a47da4588fea94
Reviewed-on: https://bazel-review.googlesource.com/1362
MOS_MIGRATED_REVID=94914935
diff --git a/src/main/protobuf/BUILD b/src/main/protobuf/BUILD
index 717d9bf..c319e34 100644
--- a/src/main/protobuf/BUILD
+++ b/src/main/protobuf/BUILD
@@ -1,11 +1,8 @@
 package(default_visibility = ["//visibility:public"])
 
-load("/tools/build_rules/genproto", "genproto")
+load("/tools/build_rules/genproto", "proto_java_library")
 
-[genproto(
-    name = "proto_" + proto_file,
-    src = proto_file + ".proto",
-) for proto_file in [
+FILES = [
     "build",
     "deps",
     "crosstool_config",
@@ -13,7 +10,12 @@
     "test_status",
     "bundlemerge",
     "xcodegen",
-]]
+]
+
+[proto_java_library(
+    name = "proto_" + s,
+    src = s + ".proto",
+) for s in FILES]
 
 filegroup(
     name = "srcs",