proper support for "isabelle build -P Pure";

--- a/src/Pure/Tools/build.scala	Wed Apr 01 12:58:44 2020 +0200
+++ b/src/Pure/Tools/build.scala	Wed Apr 01 12:59:05 2020 +0200
@@ -252,8 +252,8 @@
           }
           else Nil
 
-        if (pide && !is_pure) {
-          val resources = new Resources(sessions_structure, deps(name))
+        if (pide) {
+          val resources = new Resources(sessions_structure, deps(parent))
           val session = new Session(options, resources)
           val handler = new Handler(progress, session, name)
           session.init_protocol_handler(handler)