Fix two-hop interwiki prefix lookup reducing "foo.org" to "org"
As the code was arranged before, this could happen:
- Let's say an import is not done from de.wikinews.org but from wikinews.org.
- The code would reduce it to "org" and look for a match in the local interwiki table, obviously finding one, because there is a lot that ends with "org". Let's say it matches wiktionary.org.
- It continues to ask wiktionary.org if it knows wikinews.org. If it does, we end with "wiktionary:wikinews". This actually works (!), but is really weird.
Introduced in I89aab93.
This is not a problem in production. We don't have many source
wikis that don't have a sub-domain. The few we have are most
certainly resolved earlier via getPrefixFromInterwikiTable(),
and never reach the code touched in this patch. And even if,
it would still be a working interwiki prefix. Just a weird one.