DX: libmambapy import in build tree

[AntoinePrv]

This is an developer improvement for running libmambapy tests locally.
It reproduces a valid libmambapy folder inside the build tree.
This can latter be used with PYTHONPATH to quickly import libmambapy in any environment.

The workflow of making a full pip install of the library works well for CI, but in local
development, it can be error-prone as it also install libmamba in the environment, which then
takes precedence when running libmamba and mamba integration tests.

[jjerphan]

I never have had too much problem in the past but if it improves your workflow, I am not against making lifes easier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3916   +/-   ##
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  Coverage        ?   63.40%           
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  Files           ?      300           
  Lines           ?    37156           
  Branches        ?     2782           
=======================================
  Hits            ?    23557           
  Misses          ?    13546           
  Partials        ?       53           

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