Use _command_exists everywhere

[gaelicWizard]

Description

Use _command_exists instead of command -v xx &>/dev/null (and similar) throughout the code base.

Motivation and Context

I grep'd for type and command -v and declare -F and fixed everything I could find. This closes #1632.

How Has This Been Tested?

All tests pass, but real testing hasn't been done as of this writing.

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.

[gaelicWizard]

Rebased on current master and ready for review. 😃

[gaelicWizard]

@cornfeedhobo you said you might have time this weekend, so here's a PR 😆

[cornfeedhobo]

@gaelicWizard I like the spirit of this PR, but I'm not comfortable merging it since there are so many other changes that are unrelated. I'm not going to block the PR in case @NoahGorny or @davidpfarrell want to move this forward, but I would prefer to limit these type of horizontal clean-ups to a singular change, because it spans so many files. If you'd like to open up another PR that only changes which/command to _command_exists, I'll happily test and review that, and then you could submit these other changes in individual PRs.

As always, thanks so much and let me know if I can help somehow.

[davidpfarrell]

Hi All !

So @cornfeedhobo is right there really appears to be a lot of unrelated changes here.
I'm guessing this is a stacked PR built on another PR?

(assuming yes)

I think it might be best to tag ALL stacked PRs as DRAFT and untag PRs one at a time as they become clean and ready for [final] review. Of course casual reviews can happen on DRAT PRs too as reviewer's time permits ...

[edit] Clarified final vs casual reviews ...

[gaelicWizard]

Sounds fair! I'll split this up 👍

[gaelicWizard]

@cornfeedhobo, @davidpfarrell, I've split out all the files that had more than just _command_exists and I'll submit those separately. This one should be good to go now! 👍

[cornfeedhobo]

I like it! Thank you for the hard work!!

cc @NoahGorny

[NoahGorny]

well done @gaelicWizard, and of course thanks for the review @cornfeedhobo ❤️