ExtractDependencies: correctly record inheritance on type alias

This fix the test "Extracted source dependencies from public members"
which previously failed with:
  Set('G, 'E) is not equal to Set('B, 'E) (DependencySpecification.scala:34)

`H` extends `G.T[Int]` which is an alias of `B`, so the
`topLevelInheritanceDepndencies` of `H` should contain `B`, this was not
the case before because we didn't dealias before looking for the
top-level class of the dependency, so we ended up with `G`, the
top-level class in which the alias `T` is contained.

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -200,7 +200,7 @@ private class ExtractDependenciesCollector(implicit val ctx: Context) extends tp
         addDependency(ref.symbol)
         usedTypeTraverser.traverse(ref.tpe)
       case t @ Template(_, parents, _, _) =>
-        t.parents.foreach(p => addInheritanceDependency(p.tpe.typeSymbol))
+        t.parents.foreach(p => addInheritanceDependency(p.tpe.classSymbol))
       case _ =>
     }
     traverseChildren(tree)