Stop depending on scala-reflect

[smarter]

After the hyper bootstrap and #2062, the remaining uses of scala-reflect are:

• scala.reflect.io.*, should be replaced by our own abstraction layer (@felixmulder is working on one).
• scala.reflect.internal.util.WeakHashSet used only in the backend, should be copy-pasted into the dotty sources.

[smarter]

Fixed a while ago.

[allanrenucci]

Do you mean it can be removed from: https://github.com/lampepfl/dotty/blob/211cc8f1d0d06a45a70e6b7b4c6e5a9468abb5d4/project/Build.scala#L1167

[smarter]

This one is just a dummy we create in our build, not the real scala-reflect that we used to depend on.