Refactor SymUtils

 - Move to core package. It was for historical reasons in transform because
   it was originally intended as a collection of type operations that were
   were useful in transform phases. But it's now used from everywhere.
 - Make a base class of Types, so that it does not need to be imported explicitly.

Also: move isDerivedValueClass to SymUtils

[Cherry-picked 125321e][modified]

compiler/src/dotty/tools/dotc/core/SymUtils.scala

@@ -377,8 +377,6 @@ class SymUtils:
       self.hasAnnotation(defn.ExperimentalAnnot)
       || isDefaultArgumentOfExperimentalMethod
       || (!self.is(Package) && self.owner.isInExperimentalScope)
-      || self.topLevelClass.ownersIterator.exists(p =>
-          p.is(Package) && p.owner.isRoot && p.name == tpnme.dotty)
 
     /** The declared self type of this class, as seen from `site`, stripping
     *  all refinements for opaque types.