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 75.43%. Comparing base (85f4ebc) to head (34b3e4d).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           refactor-plugin-registration     #788   +/-   ##
=============================================================
  Coverage                         75.43%   75.43%           
=============================================================
  Files                               296      296           
  Lines                             25591    25593    +2     
=============================================================
+ Hits                              19305    19307    +2     
  Misses                             6286     6286           

Flag	Coverage Δ
unittests	75.43% <100.00%> (+<0.01%)	⬆️

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