string 'unsatisfied' and whose second element is a requirement
which couldn't be satisified.
+ In the set of :class:`Distribution` instances returned, some
+ attributes will be set:
+ * The instance representing the passed-in ``requirement`` will
+ have the :attr:`requested` attribute set to ``True``.
+ * All requirements which are not installation requirements (in
+ other words, are needed only for build and test) will have
+ the :attr:`build_time_dependency` attribute set to ``True``.