Fix Starlark errors in protobuf.bzl in newer bazel versions

In newer bazel, `load()` statements have to be at the top.

The `REPOSITORY_NAME` and `PACKAGE_NAME` constants are also no longer
available. This patch replaces those with the equivalent `native.*`
function calls.

Change-Id: I072d5cd380053616546f34256e55adfd547e8d00
diff --git a/third_party/protobuf/protobuf.bzl b/third_party/protobuf/protobuf.bzl
index 7925d18..a125606 100644
--- a/third_party/protobuf/protobuf.bzl
+++ b/third_party/protobuf/protobuf.bzl
@@ -1,3 +1,5 @@
+load("@//tools/build_rules:select.bzl", "compiler_select")
+
 def _GetPath(ctx, path):
   if ctx.label.workspace_root:
     return ctx.label.workspace_root + '/' + path
@@ -174,8 +176,6 @@
     "/wd4514", # -Wno-unused-function
 ]
 
-load("@//tools/build_rules:select.bzl", "compiler_select")
-
 COPTS = [
         "-DHAVE_PTHREAD",
         "-DGOOGLE_THIRD_PARTY_PROTOBUF",
@@ -302,8 +302,8 @@
   Args:
     srcs: the well known protos
   """
-  root = Label("%s//protobuf_java" % (REPOSITORY_NAME)).workspace_root
-  pkg = PACKAGE_NAME + "/" if PACKAGE_NAME else ""
+  root = Label("%s//protobuf_java" % (native.repository_name())).workspace_root
+  pkg = native.package_name() + "/" if native.package_name() else ""
   if root == "":
     include = " -I%ssrc " % pkg
   else: