Remove registry preload and avoid updating sources prematurely instead

Remove registry preload and avoid updating sources prematurely instead

So registry preloading was added[1] to avoid updating the root path
multiple times unnecessarily, since that's an expensive operation. But
with Package::from_path, we can get the root package just from a
manifest and a config without having to update the whole path source.

So now we don't have to remember to preload the registry if we've
already updated a source and want to use the registry later. Instead, we
just avoid loading the source initially and let the registry do it when
it needs to -- which is now in resolve_with_previous.

This does cause one tested error message to happen slightly later than
it used to, but it still happens.

[1] - ef8c651af