Remove import time plugin loading and dynamic modules

[Helveg]

Describe the work done

Simplified import time code complexity, removed most of it.

List which issues this resolves:

closes #815

Tasks

• [x] Added tests

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📚 Documentation preview 📚: https://bsb--829.org.readthedocs.build/en/829/

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.05263% with 55 lines in your changes are missing coverage. Please review.

Project coverage is 84.84%. Comparing base (5b21255) to head (3c06517).

Files	Patch %	Lines
bsb/config/__init__.py	37.31%	42 Missing :warning:
bsb/cli/commands/_commands.py	25.00%	3 Missing :warning:
bsb/storage/__init__.py	91.17%	3 Missing :warning:
bsb/__init__.py	94.44%	1 Missing :warning:
bsb/connectivity/import_.py	0.00%	1 Missing :warning:
bsb/core.py	80.00%	1 Missing :warning:
bsb/morphologies/parsers/parser.py	66.66%	1 Missing :warning:
bsb/options.py	96.15%	1 Missing :warning:
bsb/placement/import_.py	50.00%	1 Missing :warning:
bsb/services/pool.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #829 +/- ## ========================================== - Coverage 84.85% 84.84% -0.01% ========================================== Files 104 104 Lines 14057 13982 -75 ========================================== - Hits 11928 11863 -65 + Misses 2129 2119 -10 ```

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