Sometimes source packages reference files already present in the pool.
Allow for those file to be omitted when importing packages either via
`repo add` or `repo include`. If file is missing, aptly would make
an attempt to look up file in the package pool (by checksum) and
use it.
Fixes: #278
This is related to #506
As a first step, don't pass MD5 explicitly, pass checksum info object,
so that as a next step we can choose which hash to use.
There should be no functional changes so far.
Next step: stop returning explicit paths from public package pool.
The dependency specified when looking for conflicting packages
does not take into account the package version, as by default the
Relation field in the Dependency structure is set to VersionDontCare.
This should fix#185.
