Move six.BUILD to third_party and update paths

Makefile.am

@@ -1236,7 +1236,7 @@ EXTRA_DIST = $(@DIST_LANG@_EXTRA_DIST)   \
   python/release/wheel/Dockerfile                \
   python/release/wheel/protobuf_optimized_pip.sh \
   python/release/wheel/README.md         \
-  six.BUILD                              \
+  third_party/six.BUILD                  \
   third_party/zlib.BUILD                 \
   util/python/BUILD
 

WORKSPACE

@@ -18,7 +18,7 @@ protobuf_deps()
 
 http_archive(
     name = "six",
-    build_file = "@//:six.BUILD",
+    build_file = "@//:third_party/six.BUILD",
     sha256 = "d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73",
     urls = ["https://pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz"],
 )

protobuf_deps.bzl

@@ -25,7 +25,7 @@ def protobuf_deps():
     if not native.existing_rule("six"):
         http_archive(
             name = "six",
-            build_file = "@//:six.BUILD",
+            build_file = "@com_google_protobuf//:third_party/six.BUILD",
             sha256 = "d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73",
             urls = ["https://pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz"],
         )