Revert "Use sitecustomize to emulate venv during python install"

[cottsay]

This reverts #512.
Fixes #518.

There is a conditional in setuptools that causes some setup.cfg options to be ignored while in a virtual environment. While their use is probably incorrect, the anitpattern is too widespread to ignore, and we can't see a way to avoid the conditional in setuptools while still using the sitecustomize approach to work around the platform patches.

This will regress colcon builds on Fedora 36, homebrew, and Jammy with setuptools installed from pip once again, until we can come up with a new solution.

[codecov]

Codecov Report

Merging #520 (cd94cc0) into master (d02e374) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   81.68%   81.66%   -0.03%     
==========================================
  Files          61       60       -1     
  Lines        3570     3566       -4     
  Branches      685      685              
==========================================
- Hits         2916     2912       -4     
  Misses        602      602              
  Partials       52       52              

Impacted Files	Coverage Δ
colcon_core/task/python/build.py	36.02% <50.00%> (-1.02%)	⬇️

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[nuclearsandwich]

It is unfortunate but this appears to be case where the cure is imperfect and more insidious than the disease, even if not strictly speaking "worse".

[cottsay]

Partially for my own sake, this is the spot in setuptools where this change originally went wrong: https://github.com/pypa/setuptools/blob/f777a40ed9abf529906c2939f80a184a5ed035fa/setuptools/dist.py#L691

It actually exposes a general problem here that I'm not sure we will be able to solve: If you install colcon in a venv, you will always trigger the bug #518. Evidently it wasn't a wide enough use case to get our attention before I tried to use the venv-like approach here to accomplish our goals, but here we are.

The workaround of using underscores mentioned in #518 probably only works accidentally - a comment in the setuptools code indicates that ignoring the entries is an attempt to disable the customized installation directories.