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.

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index 124ddd0..b3f9e1c 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -127,6 +127,7 @@
 # ========= Packaging rules =========
 
 exports_files([
+    "packaging.bzl",
     "pip.bzl",
     "whl.bzl",
 ])