Rewrite //:combine_distfiles.sh in Python

It can only pack to zip for now (packing to tar is
not trivial and I haven't figured it out yet).

This allows building //:bazel-distfile on Windows.
Previously it was either timing out or taking so
long that it was unbearable (over 10 minutes).
I never waited long enough to see it build.

The new Python version runs under just a few
seconds.

Change-Id: I3264eb7132dd58c581c4216e5bbab035a79d716d
PiperOrigin-RevId: 164954162
diff --git a/src/BUILD b/src/BUILD
index d788f1f..ce62eae 100644
--- a/src/BUILD
+++ b/src/BUILD
@@ -125,6 +125,7 @@
 py_library(
     name = "create_embedded_tools_lib",
     srcs = ["create_embedded_tools_lib.py"],
+    visibility = ["//:__pkg__"],
 )
 
 py_binary(