bridge: Add cockpit-bridge --packages to list packages

This command line tool lists the installed packages, their
paths, checksums and aliases.
bridge: Fix checksum issues with packages

 * Make order of finalizing the checksums irrelevant
 * If a dependent package isn't cacheable or doesn't exist
   then don't allow a checksum

Closes #1350
Signed-off-by: Marius Vollmer <mvollmer@redhat.com>
 - Fixed memory leaks