restructure plugins

[DirkEilander]

Issue addressed

Fixes #958 Fixes #960

Explanation

The entrypoints should now expose a submodule/script with a __hydromt_eps__. Since this is more specific than the python all variable it poses less restrictions on the package structure. The __hydromt_eps__ should be a list of strings with names of classes that are exposed. The name of each class is first taken from the name attribute and otherwise the class name itself from __name__. Note that both Driver and Model have a name attribute, ModelComponent does not (yet) have this attribute. In addition I added a check to make sure the exposed classes are nonabstract subclasses of the base class that is associated with each entrypoint group. Each entrypoint group now has it's own class to avoid code duplication.

General Checklist

• [x] Updated tests or added new tests
• [x] Branch is up to date with main
• [x] Tests & pre-commit hooks pass
• [ ] Updated documentation
• [x] Updated changelog.rst

Data/Catalog checklist

• [ ] data/catalogs/predefined_catalogs.yml has not been modified.
• [ ] None of the old data_catalog.yml files have been chagned
• [ ] data/chagnelog.rst has been updated
• [ ] new file uses LF line endings (done automatically if you used update_versions.py)
• [ ] New file has been tested locally
• [ ] Tests have been added using the new file in the test suite

Additional Notes (optional)

Add any additional notes or information that may be helpful.

[DirkEilander]

The migration guide has been updated in #950 to avoid merge conflicts