Add support for plugin directories

[Schamper]

General idea is that you create your plugin in a file called _plugin.py in a certain directory. That directory can then hold other files such as specific helpers. The plugin path/name is then derived from the directory path up until the module name, instead of including it.

For example:

apps/something/myapp
- _plugin.py
- helpers.py

The plugin will register under the apps.something.myapp path.

Closes #890.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.23%. Comparing base (9d13f89) to head (39c9ca9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #788   +/-   ##
=======================================
  Coverage   78.23%   78.23%           
=======================================
  Files         336      336           
  Lines       29664    29665    +1     
=======================================
+ Hits        23208    23209    +1     
  Misses       6456     6456           

Flag	Coverage Δ
unittests	78.23% <100.00%> (+<0.01%)	⬆️

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