Deprecate and remove lib.bzl

This process was started in commit 1099dd2 by @thomasvl .

1. Use a `print()` warning.
1. Wait one or two releases. ← we are here.
1. Use a `fail()` error.
1. Remove `lib.bzl`

Start by reviewing commit 1099dd2 and the current lib.bzl file. Check the release history to confirm that the warning period has passed, then follow the staged path in the issue from print() warning to fail() error and removal. Done means lib.bzl is removed after the deprecation transition.

Written by the indexing model from the issue text.