pmb.parse.depends: ignore conflict dependency that no longer exists

So it looks like some packages might be mentioned in the "depends=" of other packages and marked as conflict, for example: depends="!foo"

..and the conflicting package can be dropped from the repo entirely if it is no longer needed, however the reference to it in depends= still exists. This handles that situation by just ignoring the missing package if it is only a conflicting dependency.

fixes pmaports#1525 (closed)