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.