permanently redirect /packages/foo to /package/foo

Fixes https://github.com/npm/newww/issues/98
we should probably leave `/{version?}` in here, as per https://github.com/npm/newww/blob/master/facets/registry/show-package.js#L19-L21.

The trouble is that right now, if you go to something like https://www.npmjs.org/package/npm-expansions/1.6.0, it actually *does* go somewhere. We'll want to do that again eventually in newww, but since we're not saving the version-specific readme, we're discouraging it (for now). That said, we don't want the whole thing to break if someone adds a version number, either.