completions: clear and consistent log messages

[gaelicWizard]

Description

Many plugins/completions/whatever check if their associated command is already installed before loading. Find, fix, and add in/to all the completions to ensure consistency and improve bash-it doctor.

Since completions are such a focused set of functions, create some helpers to simplify.

Motivation and Context

The _command_exist() (et al) functions were all weird due to having logging shoved in sideways, which was neither informative nor used anywhere. This pulls the logging from the utility functions and adds it to the relevant call sites, as well as adding to several other plugins that could use it.

In particular, some completions or some plugins may be useless but harmless if their associated command is not installed. I've opted to add logging, but not to force an early short-circuit. For example, I've made improvements to a lot of the completions code recently, but I don't have all the relevant tools installed. I load the completion and can test it out anyway, so I don't want to unnecessarily short-circuit it.

The result of all this should be a more reasonable output when $BASH_IT_LOG_LEVEL is set, and from bash-it doctor, as well as avoid polluting theme variables that check for installed utilities.

How Has This Been Tested?

Types of changes

• Enhancement (non-breaking change which makes things better)

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.

[seefood]

Howdie! please merge from master and resolve conflicts so we can move it to review :)