Find a distribution and all distributions it depends on.

        :param requirement: The requirement specifying the distribution to
                            find, or a Distribution instance.
        :param meta_extras: A list of meta extras such as :test:, :build: and
                            so on.
        :param prereleases: If ``True``, allow pre-release versions to be
                            returned - otherwise, don't return prereleases
                            unless they're all that's available.

        Return a set of :class:`Distribution` instances and a set of
        problems.

        The distributions returned should be such that they have the
        :attr:`required` attribute set to ``True`` if they were
        from the ``requirement`` passed to ``find()``, and they have the
        :attr:`build_time_dependency` attribute set to ``True`` unless they
        are post-installation dependencies of the ``requirement``.

        The problems should be a tuple consisting of the string
        ``'unsatisfied'`` and the requirement which couldn't be satisfied
        by any distribution known to the locator.

def find(self, requirement, meta_extras=None, prereleases=False):
        """
        Find a distribution and all distributions it depends on.

        :param requirement: The requirement specifying the distribution to
                            find, or a Distribution instance.
        :param meta_extras: A list of meta extras such as :test:, :build: and
                            so on.
        :param prereleases: If ``True``, allow pre-release versions to be
                            returned - otherwise, don't return prereleases
                            unless they're all that's available.

        Return a set of :class:`Distribution` instances and a set of
        problems.

        The distributions returned should be such that they have the
        :attr:`required` attribute set to ``True`` if they were
        from the ``requirement`` passed to ``find()``, and they have the
        :attr:`build_time_dependency` attribute set to ``True`` unless they
        are post-installation dependencies of the ``requirement``.

        The problems should be a tuple consisting of the string
        ``'unsatisfied'`` and the requirement which couldn't be satisfied
        by any distribution known to the locator.
        """

        self.provided = {}
        self.dists = {}
        self.dists_by_name = {}
        self.reqts = {}

        meta_extras = set(meta_extras or [])
        if ':*:' in meta_extras:
            meta_extras.remove(':*:')
            # :meta: and :run: are implicitly included
            meta_extras |= set([':test:', ':build:', ':dev:'])

        if isinstance(requirement, Distribution):
            dist = odist = requirement
            logger.debug('passed %s as requirement', odist)
        else:
            dist = odist = self.locator.locate(requirement,
                                               prereleases=prereleases)
            if dist is None:
                raise DistlibException('Unable to locate %r' % requirement)
            logger.debug('located %s', odist)
        dist.requested = True
        problems = set()
        todo = set([dist])
        install_dists = set([odist])
        while todo:
            dist = todo.pop()
            name = dist.key     # case-insensitive
            if name not in self.dists_by_name:
                self.add_distribution(dist)
            else:
                #import pdb; pdb.set_trace()
                other = self.dists_by_name[name]
                if other != dist:
                    self.try_to_replace(dist, other, problems)

            ireqts = dist.run_requires | dist.meta_requires
            sreqts = dist.build_requires
            ereqts = set()
            if meta_extras and dist in install_dists:
                for key in ('test', 'build', 'dev'):
                    e = ':%s:' % key
                    if e in meta_extras:
                        ereqts |= getattr(dist, '%s_requires' % key)
            all_reqts = ireqts | sreqts | ereqts
            for r in all_reqts:
                providers = self.find_providers(r)
                if not providers:
                    logger.debug('No providers found for %r', r)
                    provider = self.locator.locate(r, prereleases=prereleases)
                    # If no provider is found and we didn't consider
                    # prereleases, consider them now.
                    if provider is None and not prereleases:
                        provider = self.locator.locate(r, prereleases=True)
                    if provider is None:
                        logger.debug('Cannot satisfy %r', r)
                        problems.add(('unsatisfied', r))
                    else:
                        n, v = provider.key, provider.version
                        if (n, v) not in self.dists:
                            todo.add(provider)
                        providers.add(provider)
                        if r in ireqts and dist in install_dists:
                            install_dists.add(provider)
                            logger.debug('Adding %s to install_dists',
                                         provider.name_and_version)
                for p in providers:
                    name = p.key
                    if name not in self.dists_by_name:
                        self.reqts.setdefault(p, set()).add(r)
                    else:
                        other = self.dists_by_name[name]
                        if other != p:
                            # see if other can be replaced by p
                            self.try_to_replace(p, other, problems)

        dists = set(self.dists.values())
        for dist in dists:
            dist.build_time_dependency = dist not in install_dists
            if dist.build_time_dependency:
                logger.debug('%s is a build-time dependency only.',
                             dist.name_and_version)
        logger.debug('find done for %s', odist)
        return dists, problems