Improvement move pkg cache fs calls to repo plugin

[vanridal]

This PR is an attempt to decouple the package cache operation to the repository plugins
First commit moves only the copytree function call to the base repo class
While the second commit moves other fs calls like os.path.isdir and os.stat to the variant resource class

This can allow users to customize how the package cache touches the filesystem on a per repository bases either via the cache_variant() call, or on the variant resource

Does this help with the artifact repo direction, could some remote repos use the current cache system via moving these functions to the repository plugin?

I personally would like a way to customize pkg cache operation via a custom filesystem plugin to optimize cache speeds

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.12%. Comparing base (dac6bcb) to head (a8ff4bc).

Files with missing lines	Patch %	Lines
src/rez/package_cache.py	66.66%	1 Missing ⚠️
src/rez/package_resources.py	80.00%	1 Missing ⚠️
src/rez/packages.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1993      +/-   ##
==========================================
+ Coverage   60.11%   60.12%   +0.01%     
==========================================
  Files         163      163              
  Lines       20098    20109      +11     
  Branches     3494     3494              
==========================================
+ Hits        12082    12091       +9     
- Misses       7205     7207       +2     
  Partials      811      811              

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