fetch: rename 'tarball' to 'download'
Adjust semantics of Formula#fetch

It doesn't really make logical sense that this method returns both the
fetched path (or sometimes nil!) and the downloader, so just return the
path (again, or nil!) and callers that want the downloader can ask for
it separately.
Formula#fetch: always return the cached path

This allows us to remove some type checks that were protecting against
potential nils.