No existentials

[olafurpg]

Test from srewrite

-  val w: Wrapper[TypArg] forSome { type TypArg >: Bar <: Foo } = ???
+  val w: existentials.Wrapper[_ >: existentials.Bar <: existentials.Foo] = ???

   val doNothingHere: Wrapper[_ >: Bar /* troll comment */ <: Foo] = ???

-  val p1: Pair[A, B] forSome {
-    type A <: Bar
-    type B >: Foo
-  } = ???
+  val p1: existentials.Pair[_ <: existentials.Bar, _ >: existentials.Foo] = ???

[olafurpg]

Migration rules for Dotty will live in a different directory than the scalacenter/scalafix so that they can evolve at a different pace than Scalafix APIs. Thanks to recent improvements, custom rules can still have the identical UX as built-in rules.

• automatically installed by sbt-scalafix
• tab completions in the sbt shell like built-in rules
• documentation on the main Scalafix website