Add version aliases

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, the pyenv-alias plugin broke when the code was updated to
allow specifying multiple versions of Python to install at once, and the
current code is not amenable to modifying the version in place with another
plugin.

2.4.1

2.4.0

v2.3.30