'aptly publish search' request

[quentinsf]

Following on from this discussion, it would be great to find out the list of packages currently published by aptly, whether from repos or snapshots.

Andrey suggested that a 'search' command would be the best way to span the two rather different cases of repos and snapshots consistently.

My particular use case is that, after publishing, I need to provide a list of packages to another system where users must only be able to select from packages that are actually available!

The ideal output for me would be machine-readable and would include distribution, component, package name, architecture and version number as separate fields.

Sorry I don't yet know enough Go to submit a useful Pull Request :-)