[BUGFIX] Make getPackageOfObject() work for proxy objects
The method ``PackageManager::getPackageOfObject()`` failed to
resolve the package of a given object if it was a proxy.
This change fixes this by comparing the namespace of the object
with the package namespaces rather than the file locations.
The Flow/Doctrine proxy classes usually reside in a folder underneath
``Data/Temporary``. Comparing those paths with the package root paths
led to invalid results.
Releases: master, 2.0, 2.1