Simplify resolving symlinks: replace native extension with pwd -P - #3389

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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)
  • N/A

Description

• Here are some details about my PR

This method is used in Rbenv (rbenv/rbenv#1428) and Homebrew (https://github.com/Homebrew/brew/blob/fa5368edd38eb8d43f8cfcca15c7cfb83ee5a78d/Library/Homebrew/brew.sh#L76-L78).

pwd -P is portable between Linux and BSD (MacOS), unlike readlink -f.

Tests

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