Python: Update BUILD.bazel files.

This allows us to (later) build the whole python language pack with
bazel.

python/downgrades/BUILD.bazel

@@ -0,0 +1,12 @@
+load("@rules_pkg//:mappings.bzl", "pkg_files", "strip_prefix")
+
+pkg_files(
+    name = "downgrades",
+    srcs = glob(
+        ["**"],
+        exclude = ["BUILD.bazel"],
+    ),
+    prefix = "downgrades",
+    strip_prefix = strip_prefix.from_pkg(),
+    visibility = ["//python:__pkg__"],
+)