refactor: use lazy_loader with subpackages

[nstarman]

This follows the normal usage of galax, e.g. import galax.dynamics as gd.
The current format destroys the lazy import right away.

Unfortunately scientific-python/lazy-loader#94 means we can't do star imports in the stub files. When that's resolved everything will be a lot simpler.

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (c972652) 88.44% compared to head (66d8ac4) 88.43%.
Report is 7 commits behind head on main.

Files	Patch %	Lines
...amics/_dynamics/mockstream/mockstream_generator.py	72.22%	5 Missing ⚠️
src/galax/dynamics/_dynamics/orbit.py	84.00%	4 Missing ⚠️
src/galax/potential/_potential/base.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
- Coverage   88.44%   88.43%   -0.01%     
==========================================
  Files          42       45       +3     
  Lines        1324     1375      +51     
==========================================
+ Hits         1171     1216      +45     
- Misses        153      159       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.