Use fqnames for SortedSet and TreeSet from java.util

Remove internal typealiases for SortedSet and TreeSet

libraries/tools/kotlin-stdlib-gen/src/templates/Snapshots.kt

@@ -101,8 +101,8 @@ object Snapshots : TemplateGroupBase() {
     } builder {
         typeParam("T: Comparable<T>")
         doc { "Returns a [SortedSet] of all ${f.element.pluralize()}." }
-        returns("SortedSet<T>")
-        body { "return toCollection(TreeSet<T>())" }
+        returns("java.util.SortedSet<T>")
+        body { "return toCollection(java.util.TreeSet<T>())" }
     }
 
     val f_toSortedSet_comparator = fn("toSortedSet(comparator: Comparator<in T>)") {
@@ -116,8 +116,8 @@ object Snapshots : TemplateGroupBase() {
                 Elements in the set returned are sorted according to the given [comparator].
             """
         }
-        returns("SortedSet<T>")
-        body { "return toCollection(TreeSet<T>(comparator))" }
+        returns("java.util.SortedSet<T>")
+        body { "return toCollection(java.util.TreeSet<T>(comparator))" }
     }
 
     val f_toMutableList = fn("toMutableList()") {