Ignore virtualenvs in `pyenv latest' in a clean way

[native-api]

Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• My PR addresses the following pyenv issue (if any)
  • Closes Fix pyenv-latest to ignore virtualenvs #2608

Description

• Here are some details about my PR

The core reason why #2608 happened is because pyenv-versions prints */envs/* in addition to the raw versions/ contents. So the clean way to fix it is to make it not do so in the first place.

Tests

• My PR adds the following unit tests (if any)
  Test for the added pyenv-versions option