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 constructed by the lazy type completer of the
enclosing class. So currently it is closing over a mutable
variable; hence the Heisenbug.

This issue was exposed by the changes in b74c33eb860,
which was introduced in Scala 2.10.1.
SI-4760 Parser handles block-ending import

Don't molest the RBRACE.

Updated with additional parse tests suggested
by @retronym.  "What are you lazy?"  Yes, I
must be lazy.  Can't ScalaCheck or Par-Test
generate these tests automatically?  That seems
like a reasonable expectation.
Merge pull request #2899 from som-snytt/issue/4760-parser

SI-4760 Parser handles block-ending import
Merge pull request #2876 from retronym/ticket/7782

SI-7782 Derive type skolems at the ground level