Bazel: Replace native {http,git}_archive with Skylark rules

Future Bazel versions will discontinue support for native http rules and
replace them with Skylark equivalents: [1]. Prepare the Gerit build tool
chain for that move.

The replacements depend on this PR in rules_closure project: [2], that
wasn't merged yet. For now we can apply "patch on import", thanks
Skylark's http rules patch support. When [2] is merged, and new version
of rules_clsoure is released, we can remove the patching procedure.

One side effect of this change is that Skylark version of http_file lost
the ability to save the downloaded artifact under its real name and is
using a generic download file name. The rule closure tool chain stumbles
on this, and rejects to accept polymer_closure artifact without ".js"
suffix. To rectify add intermediate rule and renamed "download" file to
one with js suffix "polymer_closure_renamed.js": [3].

[1] https://groups.google.com/d/topic/bazel-discuss/dO2MHQLwJF0/discussion
[2] https://github.com/bazelbuild/rules_closure/pull/270
[3] http://paste.openstack.org/show/722557

Inspired-By: "Klaus Aehlig" <aehlig@google.com>
Feature: Issue 9151
Change-Id: I44ca2ecfea67d2f4434a4743e7a6f8a3645e96a8

lib/polymer_externs/BUILD

@@ -18,9 +18,16 @@ package(
 
 load("@io_bazel_rules_closure//closure:defs.bzl", "closure_js_library")
 
+genrule(
+    name = "polymer_closure_renamed",
+    srcs = ["@polymer_closure//file"],
+    outs = ["polymer_closure_renamed.js"],
+    cmd = "cp $< $@",
+)
+
 closure_js_library(
     name = "polymer_closure",
-    srcs = ["@polymer_closure//file"],
+    srcs = [":polymer_closure_renamed"],
     data = ["//lib:LICENSE-Apache2.0"],
     no_closure_library = True,
 )