cleanup: reduce amount of boilerplate needed in main.py

[nfelt]

This cleans up our main.py to reduce boilerplate needed for the minimal functionality:

1. We migrate a few lines of global initialization (which are themselves short, but have longer comments) into a new global_init() helper in a main_lib.py library; this supersedes program.setup_environment(). (We don't want to just move everything into program since we don't want it to pick up a dependency on TensorFlow.)

2. We remove some fallback logic in main.py that handled the case where we don't have absl available. It's been a required dep for years (since Replace tf.logging with absl.logging #1654).

This is intended to simplify the process of forking main.py as is being done in #4588.