fixup! Add referred parts' hash strings in __buildout_signature__, that...

fixup! Add referred parts' hash strings in __buildout_signature__, that invokes rebuild of a part when one of its (recursive) dependencies are modified.

Comment by Julien Muchembled:

    Maybe we again have the original behaviour because of
    commit dffdeffc.

    Anyway, the behaviour of buildout at this point is meaningless.
    Buildout was previously invoked with install d3 d4 command-line
    arguments and I consider that d2 should have been reinstalled,
    because a section shall not access the option of another section
    if the latter is not properly installed.

src/zc/buildout/tests/buildout.txt

@@ -2839,7 +2839,6 @@ Now, if we run the buildout without the install command::
 
     >>> print_(system(buildout), end='')
     Develop: '/sample-buildout/recipes'
-    Uninstalling d4.
     Uninstalling d2.
     Uninstalling d1.
     Uninstalling debug.
@@ -2849,8 +2848,7 @@ Now, if we run the buildout without the install command::
     Installing d2.
     d2: Creating directory data2
     Updating d3.
-    Installing d4.
-    d4: Creating directory data2-extra
+    Updating d4.
 
 We see the output of the debug recipe, and that ``data2`` was created.  We
 also see that ``d1`` and ``d2`` have gone away::