Part of fixing #88098
## Context
This is 1/3 PRs to address issue 88098 (move label check failure logic from `check_labels.py` workflow to `trymerge.py` mergebot. Due to the messy cross-script imports and potential circular dependencies, it requires some refactoring to the scripts before, the functional PR can be cleanly implemented.
## What Changed
1. Extract extracts label utils fcns to a `label_utils.py` module from the `export_pytorch_labels.py` script.
2. Small improvements to naming, interface and test coverage
## Note to Reviewers
This series of PRs is to replace the original PR https://github.com/pytorch/pytorch/pull/92682 to make the changes more modular and easier to review.
* 1st PR: this one
* 2nd PR: https://github.com/Goldspear/pytorch/pull/2
* 3rd PR: https://github.com/Goldspear/pytorch/pull/3
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94179
Approved by: https://github.com/ZainRizvi