Order pip requirements when converting to keep a constant hash

[samson-jerome]

When using rez pip, the order of requirements collected from distributions may vary. This eventually changes the variants hash generated which leads to reinstalling a package already installed (with a different hash but identical requirements).

closes: #1414

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.52%. Comparing base (8476696) to head (41856af).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/utils/pip.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1832   +/-   ##
=======================================
  Coverage   58.52%   58.52%           
=======================================
  Files         126      126           
  Lines       17207    17207           
  Branches     3519     3519           
=======================================
  Hits        10070    10070           
  Misses       6468     6468           
  Partials      669      669           

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