Add reflect/package.scala to the blacklist to avoid cycles

Author: smarter

compiler/test/dotc/scala-collections.blacklist

@@ -1 +1,5 @@
-## The blacklist is currently empty
+## Compiling the scala.reflect package object together with other files sometimes leads to cycles,
+## but only when compiling with a non-bootstrapped dotty-library, because unpickling DottyPredef
+## requires scala.reflect.ScalaSignature which forces the scala.reflect package object.
+## Since we'll eventually be fully bootstrapped, it's not worth fixing by adding workarounds.
+scala/reflect/package.scala