Rework of module inheritance

[da-h]

This MR rewrites the way in which modules define inheritance.

In more Detail

• the .default keyword was misleading in multiple ways and has been removed (Closes Replace default as special name with boilerplate inheritance #44)

• enabling submodules (modules with modules beneath it)

• changing the way relative modules are referenced. (Closes set_scope() with relative paths #29)
  Considering the register method of the module a.b.c.d.

• test cases checking all of those features

  Before this MR:

  • .addional referenced a.b.c.additional (as typically one would use .default module constructs)
  • .. referenced a.b
  • . referenced a.b.c

  After this MR:

  • .addional references a.b.c.d.additional (as we allow submodules now)
  • .. references a.b.c
  • . is a noop.