Use rules_pkg to create distributions (#234)

This adds a packaging target to create a Bazel Federation-compliant distribution.

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index 42679ee..cb092f8 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -28,6 +28,15 @@
 
 licenses(["notice"])  # Apache 2.0
 
+filegroup(
+    name = "distribution",
+    srcs = glob(["**"]) + [
+        "//python/constraints:distribution",
+        "//python/runfiles:distribution",
+    ],
+    visibility = ["//:__pkg__"],
+)
+
 # ========= Core rules =========
 
 exports_files([