One of the pyenv-virtualenv-set-up aliases depends on a symlink.
It seems pyenv runs the bash script from such a path and it fails
because of a failing comparison (VIRTUAL_ENV not detected).

The VIRTUAL_ENV is ensured to be fully resolved as well for safety.

This requires readlink from GNU coreutils but all supported platforms
have it by default.

Extra comments included, as well as simplification of directory
detection - readlink handles this (not that `bin` itself was
ever a symlink...).

Closes-Bug: #1903887
Co-Authored-By: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Change-Id: I2fe6eb13ce7be68d346b1b3b7036859f34c896c4
(cherry picked from commit aaab1d1b)