WIP: Add version aliases

[kmsquire]

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.
    • This is a modification of an existing plugin (pyenv-build)
• 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.
    • Not applicable; this modification happens in the pyenv-build plugin, which does not exist upstream
• My PR addresses the following pyenv issue (if any)
  • Closes Install python version under a different name #218

Description

• Here are some details about my PR

An older plugin, pyenv-alias, allowed naming the desired python version with an alias, which allowed installing multiple versions of the same python version. This is particularly useful on Apple silicon if you want to install both arm and x86_64 versions of python.

Unfortunately, this plugin broke when the code was modified to allow multiple versions to be specified at once on the command line, and the updated code does not allow non-invasive modification of the version number.

Tests

• My PR adds the following unit tests (if any)

  • None

[kmsquire]

I just realized that I didn't add documentation. If this functionality looks reasonable to add, I'll be sure to do so!

[native-api]

I really don't like the idea of having to supply an argument via an envvar -- but can't readily recall anything better.

I guess we should do a brainstorm in a discussion. Someone must have been solving the same problem before!

An idea from the top of my head is an array option -- one that can be specified multiple times.
Another is to somehow pair arguments and the options.

[kmsquire]

I'm okay with either an array option, or pairing the arguments and options. I think the array option would be easier to implement.

[kmsquire]

So it turns out that I broke non-aliased builds in this. I'll look into it.

[native-api]

Got a nice-looking idea of how to couple a version and its alias: with a colon separator. A colon shouldn't normally be present in directory names (which Pyenv-managed installation names effectively are) since it's PATH separator.