Account for stable constrictors in ExtractAPI

Constructors (and potentially, in the future, other methods) can have the stable
flag set. We need to adapt ExtractAPI to this change.

Author: odersky

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

@@ -328,7 +328,7 @@ private class ExtractAPICollector(implicit val ctx: Context) extends ThunkHolder
     } else if (sym.is(Mutable, butNot = Accessor)) {
       api.Var.of(sym.name.toString, apiAccess(sym), apiModifiers(sym),
         apiAnnotations(sym).toArray, apiType(sym.info))
-    } else if (sym.isStable) {
+    } else if (sym.isStable && !sym.isRealMethod) {
       api.Val.of(sym.name.toString, apiAccess(sym), apiModifiers(sym),
         apiAnnotations(sym).toArray, apiType(sym.info))
     } else {