Convert to rez plugin

[JeanChristopheMorinPerso]

This PR makes rez-pip a rez plugin. It will be available as a sub command called pip2.

[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.82%. Comparing base (a7d8a5d) to head (c31a267).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez_pip/cli.py	75.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   89.93%   89.82%   -0.12%     
==========================================
  Files          13       14       +1     
  Lines        1033     1061      +28     
  Branches      201      205       +4     
==========================================
+ Hits          929      953      +24     
- Misses         66       70       +4     
  Partials       38       38              

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