Support consuming bazel-diff without bzlmod

[joeljeske]

After 27adb8d, consumers using bazel-diff from source via WORKSPACE are unable to load artifacts.bzl, or repository.bzl. During the community transition to bzlmod, you should support both strategies of consumption.

Please re-add repository.bzl and artifacts.bzl with equivalent contents of MODULE.bazel

[tinder-maxwellelliott]

I am sorry for the inconvenience and I will work on fixing this soon. For now you can use a JAR directly to unblock yourself while I complete the needed work.