The introduction of Controls.2 changed the way in which imports are
resolved. This patch fix some leftovers of the old resolution scheme.
Such leftovers caused false positives in import declaration to happen
when resetting the QML/JS code model.
The code has also been simplified a little, by the removal of
duplicate code and of unused methods.

Change-Id: I90bdf7ed47fdfea7cbd8259acd7a9a968f27301b
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>