Return errors from all distro's New() functions instead of logging and
returning nil. Also, return errors instead of panicking from
NewRegistry() and NewDefaultRegistry().
WithSingleDistro() doesn't follow go's naming convention for creating
objects (New*). Rename it to NewRegistry() and rename the old
NewRegistry() to NewDefaultRegistry().
The idea is that NewRegistry() can be used to create full Registry
objects from outside the package. NewDefaultRegistry() is a convenience
function that creates a Registry with all known distros.
The current `NewRegistry` implementation allows for nil values in the
map, but this leads to subtle bugs when using the registry. This patch
enforces non-nil values by introducing additional checks before we
insert the value into the map.
The change unfortunately breaks a lot of tests and therefore it is
necessary to create additional mock: distro.
The new mock is used instead of the previous "real" implementation,
which used to contain nil values.
