SI-7782 Derive type skolems at the ground level

Rather than at the current value of `skolemizationLevel`,
which could be influenced by an in-flight existential
subtype computation.

This method is called in `PolyTypeCompleter`, which
could be called at any time.

This issue was exposed by the changes in b74c33eb860,
which was introduced in Scala 2.10.1.