optional session_base, e.g. from existing Sessions.Deps in build.scala;

--- a/src/Pure/Thy/thy_resources.scala	Sun Nov 12 20:12:10 2017 +0100
+++ b/src/Pure/Thy/thy_resources.scala	Sun Nov 12 20:17:29 2017 +0100
@@ -15,11 +15,14 @@
     options: Options,
     session_name: String,
     session_dirs: List[Path] = Nil,
+    session_base: Option[Sessions.Base] = None,
     modes: List[String] = Nil,
     log: Logger = No_Logger): Session =
   {
-    val session_base = Sessions.base_info(options, session_name, dirs = session_dirs).check_base
-    val resources = new Thy_Resources(session_base, log = log)
+    val base =
+      session_base getOrElse
+        Sessions.base_info(options, session_name, dirs = session_dirs).check_base
+    val resources = new Thy_Resources(base, log = log)
     val session = new Session(options, resources)
 
     val session_error = Future.promise[String]