[apt] Add apt-cache policy info for installed pkg

apt-cache policy information for each package can be
useful to identify the source of the installed package.
Fixes #389

Signed-off-by: Louis Bouchard <louis.bouchard@canonical.com>
Fix variable naming according to comment

Signed-off-by: Louis Bouchard <louis.bouchard@canonical.com>
Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>