modprobe: don't check if module builtin to decide if it's builtin

modprobe: don't check if module builtin to decide if it's builtin

More or less confusing message, but if module is builtin in the live
system, it doesn't mean it's builtin in the target kernel.

Instead we now check if module has a path. It don't have a path only if
it's builtin in the target or if it doesn't exist at all. The latter
should not be a problem since this code is being called from inside the
library. Anyway, put an assert to make sure we get bug reports if any
case slipped in here.