SI-7038 Deskolemize harder

Inferred method return types can contain skolems for
the methods type parameters seen from within the method
body. `Namer#typeSig` deskolemized these, replacing the
skolems with the type parameter symbols.

This failed to deskolemize the underlying type of a
by a method-local type alias.

This commit changes deskolemizeMap to normalize types
along the way.

preview by @adriaanm, probably can wait for 2.10.2 though