Create Snapshots that only contain the newest version of a given software package

[sliverc]

Some work has been done but not completed in branch:
https://github.com/smira/aptly/tree/package-version-slice

Following would need to be implemented for this to work:

• Complete implementation of VersionRangeQuery
• Package Catalog -> add method to get all pkgs with same name
• Implement sorting of debian versions (as per https://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-Version)

Issue upstream: https://github.com/smira/aptly/issues/291