PERF: refactor atef.bin cli entrypoint to defer imports (try 2)

[tangkong]

Description

A different attempt at #256 , this time making separate submodules for each sub-parser in order to avoid changes to atef.bin.main

More submodules than before, but less modifications to the existing logic.

Motivation and Context

A suggestion by Zach in #256

How Has This Been Tested?

Interactively.

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Code has been checked for threading issues (no blocking tasks in GUI thread)
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I think the extra submodules were worth it, it's interesting to see these subparser modules that only have the parser and defer the deep import until it's time to execute.

And again, this is all a compromise because Python modules are technically allowed to run as much code as they'd like on import...

[tangkong]

supercedes #256