Auto-resolve prefixes to the latest version

[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 https://github.com/pyenv/pyenv/issues/XXXX

Description

• Here are some details about my PR

When given a prefix rather than full name, Pyenv resolves it to the latest release in the corresponding version line.

CAVEATS:

• It must be a full prefix -- the actual searched prefix is <prefix>[-.]
• Other flavors are likely sorted incorrectly atm. Need someone with a use case to clarify the intended semantic.
• Prereleases and versions with some suffixes (-dev, -src, -latest) are not searched. Again, the intended semantic is unclear.
• pyenv uninstall has been excluded from the resolution feature: deleting a dynamically selected installation could be problematic. Need user input to be sure that this would be okay.

Tests

• My PR adds the following unit tests (if any)
  Tests for the new functionality in

[BrandonLWhite]

Awesome feature. This has been the missing piece in pyenv. Thank you for implementing this!