Resolve symlinks when looking for the python interpreter

[ericriff]

This makes the symlink point to the actual python exec, i.e. python3.6 instead of plain python3.
By doing this we ensure that the environment will survive a change in the default python3 version of the host system,
which can be caused by update-alternatives, an upgrade of the OS, etc.

This addresses 1974

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[ericriff]

@gaborbernat
I managed to get the symlinks resolved by replacing this abspath call with realpath. To be honest I'm not sure if this is a fix or a hack or the ramifications that this change might introduce.
But after this change virtualenv resolves the python3 symlink.

$virtualenv ~/delme/.venv            
created virtual environment CPython3.6.12.final.0-64 in 116ms
  creator CPython3Posix(dest=/home/eric/delme/.venv, clear=False, global=False)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/eric/.local/share/virtualenv)
    added seed packages: pip==20.2.3, setuptools==50.3.0, wheel==0.35.1
  activators BashActivator,CShellActivator,FishActivator,PowerShellActivator,PythonActivator,XonshActivator
$ tree ~/delme/.venv/bin
/home/eric/delme/.venv/bin
├── activate
├── activate.csh
├── activate.fish
├── activate.ps1
├── activate_this.py
├── activate.xsh
├── easy_install
├── easy_install3
├── easy_install-3.6
├── easy_install3.6
├── pip
├── pip3
├── pip-3.6
├── pip3.6
├── python -> /usr/bin/python3.6
├── python3 -> python
├── python3.6 -> python
├── wheel
├── wheel3
├── wheel-3.6
└── wheel3.6

[codecov]

Codecov Report

Merging #1979 into main will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1979   +/-   ##
=======================================
  Coverage   94.13%   94.13%           
=======================================
  Files          86       86           
  Lines        4266     4266           
=======================================
  Hits         4016     4016           
  Misses        250      250           

Flag	Coverage Δ
#tests	94.13% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/virtualenv/discovery/py_info.py	95.06% <100.00%> (ø)

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[gaborbernat]

This file should not be touched. If we do need to resolve paths we should do it within the creators, not here.

[gaborbernat]

Closing due to inactivity will re-open if you pick it up again.