Packages::DoIndex: Ugly hack to fix content negotiation

Since we don't create the static pages for ddtplangs currently,
only for polangs, but we don't discriminate between the two in
the content negotiation we might try to serve a non-existant file.

Fixing the content negoation or the static file generation will
take some time and testing. For now, work around the bug by stat'ing
the requested file in DoIndex and falling back to English in case it doesn't
exist.