Add multiple builders

[trappitsch]

• move builders into dictionary in packager
• add selection for multiple builders
• remove a fixme
• Add builder selection to initialize (defaults to rye)
• add flit and pdm to builders

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.29730% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 98.84%. Comparing base (09337ba) to head (555810f).

Files	Patch %	Lines
src/box/packager.py	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   98.92%   98.84%   -0.08%     
==========================================
  Files          19       19              
  Lines         835      868      +33     
==========================================
+ Hits          826      858      +32     
- Misses          9       10       +1     

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