Search for plugins in PYENV_DIR and PYENV_ROOT

[cornfeedhobo]

Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • This solution is not applicable to rbenv because rbenv does not bundle the build plugin (see Make python-build an independent repository. #913)
• My PR addresses the following pyenv issue (if any)
  • fixes pyenv only looks for $PYENV_ROOT/plugins, even though install and root location can differ #1696

Description

• Here are some details about my PR

  I ran into this when using basher to install pyenv and finding that pyenv install was failing for not finding pyenv-install, despite the embedded plugin. This matches my experiences with packaging for linux, where it's common to move python-build/ into share/.

Tests

• My PR adds updates the following unit tests (if any)
  • adds its own libexec to PATH
  • adds plugin bin dirs to PATH

[cornfeedhobo]

@joshfriend could you take a look at this? It's somewhat related to an old issue that you commented on last year. Thanks!

[cornfeedhobo]

Thanks!