clean up pyenv plugin

[cornfeedhobo]

This commit cleans up the pyenv plugin to follow the newer conventions of the
other *env plugins, as well as addresses the changes made to pyenv for PATH
munging.

Motivation and Context

Fixes #1882

How Has This Been Tested?

locally

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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.

[NoahGorny]

Left a comment, looks great otherwise!

[NoahGorny]

@cornfeedhobo want to revisit this?

[NoahGorny]

Nice one @cornfeedhobo

[NoahGorny]

@cornfeedhobo fix the conflicts and I'll take another look 😄

[cornfeedhobo]

@NoahGorny rebased and pushed. Sorry, I was leaving the merge to you, but I should have merged it once your review was finished. Thanks again.