SI-7285 Fix match analysis with nested objects.

The fix for SI-6146 introduced `nestedMemberType` to
enumerate sealed subtypes based on the (prefixed) type
of the scrutinee and the symbols of its sealed subclasses.
That method needed to widen `ThisType(modSym)`s to
`ModuleTypeRef(modSym)` before calling `asSeenFrom`.

However, this could lead to confused in the match analysis,
which sees `ModuleTypeRef` as distinct from singleton types
on the same modules (after all, they aren't =:=). Spurious
warnings ensued.

This commit makes two changes:

 - conditionally re-narrow the result of `asSeenFrom` in `nestedMemberType`.
 - present `a.b.SomeModule.type` as `SomeModule` in warnings emitted
   by the pattern matcher.