elements/stack.py: Disallow sources

Stack elements merely represent a logical group of elements. For this
reason, sources do not make sense.

src/buildstream/plugins/elements/stack.py

diff --git a/src/buildstream/plugins/elements/stack.py b/src/buildstream/plugins/elements/stack.py
index 97517ca..6582338 100644
--- a/src/buildstream/plugins/elements/stack.py
+++ b/src/buildstream/plugins/elements/stack.py
@@ -33,6 +33,10 @@
     # This plugin has been modified to avoid the use of Sandbox.get_directory
     BST_VIRTUAL_DIRECTORY = True
 
+    # Stack elements merely represent a logical group of elements. For this
+    # reason, sources do not make sense.
+    BST_FORBID_SOURCES = True
+
     def configure(self, node):
         pass