refactor plugin architecture

[ReimarBauer]

plugins may need modules we not have in MSS packaged, which might increase the size of the package
The navaid plugin was for that reason not packaged with MSS
https://mss.readthedocs.io/en/stable/plugins.html

currently we have only IO plugins, but also dockingwidgets maybe designed as plugins.

our flake8 checker finds

mslib/plugins/io/__init__.py:0:1: A005 the module is shadowing a Python builtin module "io"
mslib/plugins/io/csv.py:0:1: A005 the module is shadowing a Python builtin module "csv"

When we refactor this we also need a migration script for the users used msui_settings.json

A new solution should be enable not only IO plugins.

Afterwards setup.cfg needs a cleanup.

[ReimarBauer]

related #1108