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.
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