L::Tags: Parse ambigious override cases a bit better

Try first a strict package match on the override; if that succeeds we
go on checking the extra parts.  But if it fails we know we have a bad
override.  In the failed case, we can apply heuristics to determine if
the package name was good or bad.

Signed-off-by: Niels Thykier <niels@thykier.net>