refactor: move package_order to rezplugin system

[cfxegbert]

I was trying to write a new package orderer and discovered that it is not part of the rezplugin system. This refactor moves the orderers defined in package_order.py to rezplugins.package_order plugins. Function have been added to package_order.py to remain backward compatible so if a user was directly importing an orderer it should still work.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 58.02%. Comparing base (e215a77) to head (09c12c9).
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/rez/package_order.py	81.48%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1787      +/-   ##
==========================================
- Coverage   58.39%   58.02%   -0.38%     
==========================================
  Files         126      126              
  Lines       17205    17060     -145     
  Branches     3519     3490      -29     
==========================================
- Hits        10047     9899     -148     
- Misses       6491     6496       +5     
+ Partials      667      665       -2     

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[JeanChristopheMorinPerso]

Hi @cfxegbert. Can you please explain the rationale behind this change? Context is important for us. It helps us document why things were changed, by who, the pros, cons, etc.

[cfxegbert]

This was moved to the rez plugin system so a new package order could be written without having to modify the rez source code. Before a developer would have to patch the source code and apply the patch every release. By moving to the plugin system new package orders could be added without modifing the source.

[cfxegbert]

I could also add the rez plugin hooks and not move the existing package orders if you think that would be better.

[chadrik]

Scanline VFX is strongly in support of this feature.