Added label to py_wheel progress message. (#791)

python/packaging.bzl

diff --git a/python/packaging.bzl b/python/packaging.bzl
index 9ad2daf..19b5894 100644
--- a/python/packaging.bzl
+++ b/python/packaging.bzl
@@ -226,7 +226,7 @@
         outputs = [outfile, name_file],
         arguments = [args],
         executable = ctx.executable._wheelmaker,
-        progress_message = "Building wheel",
+        progress_message = "Building wheel {}".format(ctx.label),
     )
     return [
         DefaultInfo(