Workaround the broken hashCode of api.* classes

When adding a single field to `Type`, incremental compilation using
Scala 2.12 was compiling 1 file, then 12 files, then stopped. But
incremental compilation with Dotty did not stop until the whole project
was recompiled, this happened because lookups in the `refinedTypeCache`
failed, which I tracked down to sbt-contraband generating broken
hashCode implementations (sbt/contraband#119).
We work around this by using a ListMap instead of a HashMap.

Author: smarter

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

@@ -137,8 +137,11 @@ private class ExtractAPICollector(implicit val ctx: Context) extends ThunkHolder
   /** This cache is necessary to avoid unstable name hashing when `typeCache` is present,
    *  see the comment in the `RefinedType` case in `computeType`
    *  The cache key is (api of RefinedType#parent, api of RefinedType#refinedInfo).
-   */
-  private[this] val refinedTypeCache = new mutable.HashMap[(api.Type, api.Definition), api.Structure]
+    */
+  // FIXME: This should be a HashMap but cannot be until the fix for
+  // https://github.com/sbt/contraband/issues/119 is part of a released version
+  // of sbt, until then we cannot rely on the hashCode method of the api.* classes.
+  private[this] val refinedTypeCache = new mutable.ListMap[(api.Type, api.Definition), api.Structure]
 
   private[this] val allNonLocalClassesInSrc = new mutable.HashSet[xsbti.api.ClassLike]
   private[this] val _mainClasses = new mutable.HashSet[String]