Refactor import statement into commented

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Add import tests

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
fix import parsing

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Parse extends-clauses

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>