Automatically registering on `init()` is clever, but a bit too magical
and easy to get wrong, because every binary must include all distros
somewhere.
Flip this inside out: distros now have a `New()`, which returns
something that implements the `Distro` interface. The distro package
explicitly creates all of them.
This means that distros cannot import distro itself anymore, because go
forbids import cycles. This only affected `InvalidOutputFormatError`.
Return a generic error for now.
Make distros export repository information and use those in the weldr
API. This means that repos are only specified once and that the API
returns the right packages when we allow different distros.
