Add nvchecker integration as subcommand

To start making use of nvchecker for our packages, we want to have the following per repository:

• an nvchecker configuration
• nvchecker state files (old/new JSON files)

Furthermore we will want to be able to query upstream version information using a pkgctl subcommand and be able to persist changes to state files when we do a pkgctl release (optionally prevent doing that because we may only be able to bump the pkgrel).