Change the plasma module so that it's statically analysable

[ghost]

Which provides code linting etc.

I added plasma/__init__.py which exposes all the contents of the files in plasma/ that were imported separately before. All imports have been changed to from plasma import *. For now I changed Plasma.py to PlasmaStubs.py to not conflict with the engine's Plasma module. Hoikas proposed changing the engine's module instead, to _Plasma (I didn't do that because I don't have engine development set up).

I also changed the Plasma stubs to raise NotImplementedError(), further improving code linting.

[ghost]

Amended.

[ghost]

I missed converting the import Plasma... cases (e.g. import PlasmaControlKeys), which were not analysing correctly as a result. Amended now.

[ghost]

Removed line ending changes, moved change to #123

[Hoikas]

Todo before this is merged: check for any naughty assumptions in plPythonPack and verify that packed python works after repacking with this.

[ghost]

This will probably be deferred until we're on Python 3, as we may want to use stub files for the plasma module.