Remove workarounds to import packages

[visr]

In the peilbeheerst code there is code like this: 6bdcfb6.

Instead of modifying the sys.path the Python packages under src can all be installed as editable packages. In pixi.toml they are marked as such, so if you use pixi you can just directly use import ribasim_nl. This will also help the static analysis to function better.