packaging-apt-dpkg.py's _search_contents function guesses sources in use

The _search_contents function in packaging-apt-dpkg.py searches for a file in Contents.gz to determine what package provides a file, however the function contained the following code:

"for pocket in ['-updates', '-security', '-proposed', '']:"

This ended up preferring the Contents.gz file from -proposed over the one in the release pocket for artful. This caused a bad retrace as seen in bug 1712880 because libgjs0g, from -proposed, was used for the retrace instead of libgjs0f which was installed on the crashing system. I modified the list of pockets so that it now has '-proposed' last, given that most crashes won't be from users with '-proposed' enabled.

However, _search_contents is still guessing at this point as it is not aware of which pockets the system which experienced the crash had enabled. FWIW, get_file_package calls _search_contents and is called by needed_runtime_packages which is used to evaluate libraries dynamically loaded at runtime.