Debian Support

Current Behavior: Since the architectures supported by a repository are parsed from the packages list in the mongodb, architectures included in an upstream repository are not created if there are no architecture specific packages.

Expected Behavior: The architectures should be created containing only "architectures = all" type packages.

Proposed Approach:

There should be a units_deb_arches collection in the mongodb in addition to the existing units_deb_component, units_deb_release, and units_deb collections.

This would have at least two additional benefits beyond fixing this bug:

1. Parsing lists of all Packages to extract a list of one or two architectures is not good for performance.
2. It is conceptually correct. Debian repositories are built from a pool of all packages, along with meta data that will allow you to identify useful subsets of these packages using a combination of releases, components, and architectures. The database should mirror this conceptual division.