Fix race condition in FileSystemPackageRepository directory creation, modernize usage of os.makedirs

[nrusch]

The original impetus for this PR was to fix a race condition in FileSystemPackageRepository.pre_variant_install if multiple processes attempted to create a family directory in a narrow timing window.

From there, I decided to also go through and "modernize" any usage of os.makedirs to leverage the new exist_ok kwarg added in Python 3.2.

The PR is split up into 3 commits, in case you'd prefer to only integrate a subset:

1. Fix the race condition.
2. Replace rez.utils.filesystem.safe_makedirs with direct os.makedirs calls.
3. Update any remaining uses of os.makedirs to use exist_ok=True where it seems appropriate.

[nrusch]

Thanks to @mgcollie for pointing out the original race condition.

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.07%. Comparing base (d152c9a) to head (7e21324).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/package_maker.py	50.00%	1 Missing ⚠️
src/rez/pip.py	0.00%	1 Missing ⚠️
src/rez/utils/filesystem.py	50.00%	1 Missing ⚠️
src/rez/utils/py_dist.py	0.00%	1 Missing ⚠️
src/rezplugins/package_repository/filesystem.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1913      +/-   ##
==========================================
+ Coverage   60.05%   60.07%   +0.01%     
==========================================
  Files         163      163              
  Lines       20120    20096      -24     
  Branches     3507     3493      -14     
==========================================
- Hits        12084    12072      -12     
+ Misses       7216     7208       -8     
+ Partials      820      816       -4     

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[JeanChristopheMorinPerso]

Thanks @nrusch. I agree with you that the semantics of safe_makedirs seem to be almost the same as os.makedirs(path, exists_ok=True), at least according to https://github.com/python/cpython/blob/b14986c91464b06e9016678637e41d0e0192bbe8/Lib/os.py#L211-L241, which hasn't changed since at least 3.7. There are small differences, but nothing major, only improvements.