SI-6859 type macros no longer crash sbt

By mimicking alias types, macro types no longer trip a sanity check
in sbt 0.12 and 0.13: https://github.com/harrah/xsbt/blob/ef39aeb9c1ea443c17356695758a31152910c4a7/compile/interface/src/main/scala/xsbt/API.scala#L221.

This workaround makes me sad but it's the only option to make type macros
immediately useful. Principled fix pending.