process_artifacts: Refactor modules

Introduce module hierarchy for better organization and rename a few
symbols for consistency.
This is a pure refactoring change.

BUG=b:331890379
TEST=bazel test //bazel/portage/tools/process_artifacts:all

Change-Id: I707df16cd46bed561bea5bd1a4032df711f4e45c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/bazel/+/5528442
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Commit-Queue: Shuhei Takahashi <nya@chromium.org>
Tested-by: Shuhei Takahashi <nya@chromium.org>
6 files changed