Introduce new `junction` plugin origin

This merge request implements the plan as outlined in this mail.

Beyond this, there is some additional refactoring in the earlier commits.

• Some methods which were declared private are now made public, as they were already being used illegally and were required to be public for this branch as well.
• The _pluginfactory module was reorganized again, pushing more code into the PluginOrigin such that all domain specific knowledge about origins remains encapsulated in the individual PluginOrigin implementations.
• The PluginOrigin implementations have been split up into separate python files.