zodbtools: Refer to in-tree eggs via ${<name>:egg} not just name

This is similar to d67f382f (wendelin.core: Fix test build 2)

component/zodbtools/test-common.cfg

@@ -10,8 +10,6 @@ extends =
     buildout-dev.cfg
 
 parts =
-    zodbtools
-
 #   for instance
     zodbtools-python
     slapos-cookbook
@@ -21,7 +19,7 @@ parts =
 # bin/python is preinstalled with sys.path to zodbtools & friends.
 [zodbtools-python]
 <= python-interpreter
-eggs = zodbtools[test]
+eggs = ${zodbtools:egg}[test]
 
 # env.sh for zodbtools's python to be on $PATH.
 [zodbtools-env.sh]