streamline trace output of the dependency scanner

Change-Id: I0fa8cfc90c647596ddfbae04823bb870fae81012
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
move scanner determination to separate function

Preparation for fixing QBS-566.

Change-Id: I2acdb648dc9fe247458aff76caa716c79058fa84
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
let scanners operate on FileResourceBase objects

File dependencies should be scannable for dependencies too.

Change-Id: I7223ba65ee6ca02f70c8e48165652550007e91f6
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>