Add support for "--variant -1" style syntax for rez-build

[sanikache]

This pull request adds support for specifying negative variant indices (e.g., --variants -1) in rez build and copy workflows.
Negative indices are now accepted to select variants counting backward from the end, mirroring Python’s negative indexing convention.

[maxnbk]

One failing test in the jobs, that I see at a glance (I pulled linux 3.10 as a sample)

FAILED installdir/lib/python3.10/site-packages/rez/tests/test_context.py::TestContext::test_bundled - rez.exceptions.PackageCopyError: The package does not contain the variants: None

[sanikache]

Taking a look . .

[sanikache]

It's the change in package_copy.py that's breaking test_context.py::TestContext::test_bundled . It will fail for packages with no declared variants. Pushing a commit for that shortly.

[sanikache]

It looks like there is no test for copy_package on a non-variants package. I added one to cover that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.70%. Comparing base (d415b96) to head (2bdc56c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2103      +/-   ##
==========================================
+ Coverage   60.65%   60.70%   +0.04%     
==========================================
  Files         164      164              
  Lines       20584    20594      +10     
  Branches     3579     3585       +6     
==========================================
+ Hits        12485    12501      +16     
+ Misses       7224     7219       -5     
+ Partials      875      874       -1     

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[maxnbk]

Overall I'm happy, I would ask the question if maybe, since the build_process and package_copy is almost the same, if there is a sane/easy way to extract, put the variant-counting/set/invalid logic in a shared location, and call it, but the code is subtly different so perhaps it would not be completely simple? Happy to discuss.

[sanikache]

For sure, let me take a look . .