Improve model extension import

[dweindl]

Previously, it wasn't possible to import two model modules with the same name. Now this is at least possible if they are in different locations. Overwriting and importing a previously imported extension is still not supported.

In essence, we avoid using import and sys.modules, and directly import the specific files. For a single file, this works pretty neatly, but for an actual package less so.

Closes #1936

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.60%. Comparing base (4ed131b) to head (759cf39).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
python/sdist/amici/__init__.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2604      +/-   ##
===========================================
- Coverage    77.61%   77.60%   -0.02%     
===========================================
  Files          329      329              
  Lines        21701    21708       +7     
  Branches      1477     1477              
===========================================
+ Hits         16843    16846       +3     
- Misses        4847     4851       +4     
  Partials        11       11              

Flag	Coverage Δ
cpp	73.48% <85.71%> (-0.01%)	⬇️
cpp_python	34.14% <85.71%> (-0.01%)	⬇️
petab	36.95% <85.71%> (-0.02%)	⬇️
python	72.20% <85.71%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
python/sdist/amici/__init__.py	94.11% <85.71%> (-1.34%)	⬇️

... and 2 files with indirect coverage changes