Introduce new project/product property "qbsSearchPaths".

This extends preferences.qbsPath and allows to use project-specific
imports. It also deprecates the "moduleSearchPaths" property, which
becomes redundant now.

Change-Id: I66ff8aa57f6b2657211e3f4541909b2572c6d613
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Collect items from import base path.

Currently, we only look in subdirectories.

Change-Id: Ib390c98223d50740318c421622d322581edde472
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>