add target package as dev in active environment

[disberd]

This PR will add the target package as dev dependency in the active environment handled by PlutoDevMacros.
This is not done by directly calling Pkg.develop as this will always recompile the environment.
Instead, after copying the target project and manifest in the active environment, the target package is added as dev dependency to the Project and Manifest objects (in the project and manifest field of the active EnvCache).

As part of this PR, some internal functions have also been changed to update methodology.

This change should simplify locating the target package with Base.locate_package_env automatically, which will help with migrating to loading extensions with the standard approach using Base methods

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (793507b) to head (2b933b7).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #54   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          813       820    +7     
=========================================
+ Hits           813       820    +7     

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