Fix comparison of dependent function types

Dependent function types are expressed as refinements over regular function types.
These refinements need to be compared with the standard arrow rule for function
subtyping. But comparison of method refinements so far demanded equal parameter
types.

The solution is tricky since refined types lead to selections via reflection
still cannot tolerate differing parameter types since reflexive method dispatch
uses precise parameter types. That's why we apply standard arrow rule only
for refinements where the refined method exists in the underlying class.

Fixes scala#12211

Author: odersky

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

@@ -1,4 +1,4 @@
-package dotty.tools
+sdpackage dotty.tools
 package dotc
 package core