Added fallback to system executable

[rharish101]

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)
  • Resolves Pyenv interrupt command search #1210

Description

• Here are some details about my PR:

If pyenv doesn't find an executable for the current version/virtualenv, then this searches the "system" version as a fallback. Here's a motivating use case:

Black case

I have several Black's installations:

1. global by `brew install black`

2. project-specific by `pip install black`

And if I try to run black out of projects directories, I see the error:

~ black
pyenv: black: command not found

The `black' command exists in these Python versions:
  3.6.3/envs/a
  3.6.3/envs/b
  3.6.5/envs/c
  3.7.0/envs/d
  3.7.0/envs/e
  3.7.0/envs/f
  3.7.0/envs/python-in-docker
  3.7.0/envs/g

I expect to see black running as it a system command. And this is not the same as #1204 that cares about the hierarchy of Python installations.

This issue is about if a system command has the same name as one of shims commands have, then it won't be executed when it's not available in the current Python environment.

Does it make sense?

Originally posted by @extsoft in #1210 (comment)

This behaviour looks similar to one of the features added in rbenv/rbenv#1155, although I haven't gone through it properly. This is the reason why the second pre-requisite is marked.

Tests

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