Refactor to fix multiple problems

There are no functional changes in terms of the checks performed. All
the changes concern either workflow correctness or style consistency.

- We have to pip install all of requirements.txt after all, because
  pylint wants to load imported modules. Local testing failed to
  surface this problem but once merged in the main branch, pylint
  failed.

- Manual invocation failed to work as expected once merged into the
  main branch. I changed it to use a commit SHA instead of a list of
  files, and simplified the way that the list of changed files is
  obtained. (The latter is the reason for the large number of
  deletions in the Python setup job.)

- Miscellanous trivial changes for consistency reasons.

Author: mhucka