completion/pip: simplify control flow

[gaelicWizard]

Description

Short-circuit the loader rather than indenting nearly the whole file.

Alsö, assign the _pip_completion() handler directly once it's loaded so that we get out of the way once we load it.

Motivation and Context

Indenting the entirety of the logic of the file just weirds me out...it alsö makes future diff's weird, and makes any whitespace changes look like the whole file was replaced.

Alsö, this function doesn't need to run ever once the real completion is loaded.

Alsö alsö, I feel like the _command_exists _pip_completion branch of the if statement will never be taken, no matter what else is loaded already. Won't eval "$(pip completion --bash)" register the full handler itself and then we never run?

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[davidpfarrell]

I think the changes are good so I'll tentatively approve, but this logic is @NoahGorny 's pet project so I'll leave the final decision to them ...

NOTE: I like the "get out of the way" pattern !

[NoahGorny]

Nice idea and greatly done!