Add packages and modules

[RobertoRoos]

TwinCAT doesn't really have any namespacing, but it could be nice to create the notion of a module anyway, to document the contents of a folder:

.. plc:automodule:: POUs.HwModules.SomeComponent:
   :members:

[RobertoRoos]

Issue with this: this only works if you have a (or multiple) defined file roots.

Potential solution: keep two settings:

• Sources list: list of files to be processed
• Root list: list of root directories

For example loading a .plcproj file (#4) could create both a root entry and a bunch of source files.