Graduate wheel-building code out of //experimental (#418)

* Move wheelmaker from //experimental/tools into //tools.

* Move wheel-building rules from //experimental/python to //python.
Rename from wheel.bzl to packaging.bzl to avoid confusion with existing whl.bzl
Keep a stub wheel.bzl file in the old location for backwards compatibility.

* Move wheel building examples out of experimental.

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 3d89ce5..0e3cf70 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -11,10 +11,18 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+load("//python:defs.bzl", "py_binary")
+
 package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
+# Implementation detail of py_wheel rule.
+py_binary(
+    name = "wheelmaker",
+    srcs = ["wheelmaker.py"],
+)
+
 # These files are generated and updated by ./update_tools.sh
 exports_files([
     "piptool.par",
@@ -25,6 +33,7 @@
     name = "distribution",
     srcs = [
         "BUILD",
+        "wheelmaker.py",
     ] + glob([
         "*.par",
     ]),