SI-7240 fixes language feature lookup

As I discovered today, Definitions.getMember have a fallback clause,
which accounts for the phases which have inner classes flattened.

This fallback uses nme.flattenedName to compute a flattened name, but
unfortunately nme.flattenedName produces a TermName, not a TypeName,
which means that the fallback will commence search in a wrong namespace
with predictable results.

The commit also changes another usage of nme.flattenedName in a type name
context. That one was correctly converting a TermName result to TypeName,
so this is not a bugfix, but just a refactoring for the sake of being
consistent.
Merge pull request #2234 from scalamacros/ticket/7240

SI-7240 fixes language feature lookup