wendelin.core: Fix test build 2

Always reference to in-tree eggs via ${<name>:egg} instead of just name. This fixes e.g. the following build failure:

    Installing wendelin.core-python.
    While:
      Installing wendelin.core-python.
      Base installation request: 'wendelin.core[test]', 'pygolang[pyx.build]', 'neoppod[tests]', 'ZEO[test]'
      Getting distribution for 'ZEO[test]'.
    Error: Picked: ZEO = 5.2.3

Not sure why it worked before.

Neoppod is a bit special - leave it as is for now.

component/wendelin.core/test-common.cfg

@@ -16,8 +16,6 @@ parts =
 # keep neoppod first and in parts so that ZODB is built correctly
     neoppod
 
-    wendelin.core
-
 #   for instance
     wendelin.core-python
     slapos-cookbook
@@ -29,14 +27,14 @@ parts =
 depends +=  ${scipy:egg}
 
 
-# bin/python s python interpreter with wendelin.core and all other eggs.
+# bin/python is python interpreter with wendelin.core and all other eggs.
 [wendelin.core-python]
 <= python-interpreter
 eggs =
-    wendelin.core[test]
-    pygolang[pyx.build]
+    ${wendelin.core:egg}[test]
+    ${pygolang:egg}[pyx.build]
     neoppod[tests]
-    ZEO[test]
+    ${ZEO:egg}[test]
 
 # env.sh for that python + go to be on $PATH
 [wendelin.core-env.sh]