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

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 59.31%. Comparing base (233b82c) to head (f482b34).

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 ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1913      +/-   ##
==========================================
+ Coverage   59.28%   59.31%   +0.02%     
==========================================
  Files         126      126              
  Lines       17218    17203      -15     
  Branches     3017     3008       -9     
==========================================
- Hits        10208    10204       -4     
+ Misses       6325     6316       -9     
+ Partials      685      683       -2     

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