ns that are in the project's "plugin directory" or
        directories. The `full_env`, if supplied, should be an ``Environment``
        contains all currently-available distributions.  If `full_env` is not
        supplied, one is created automatically from the ``WorkingSet`` this
        method is called on, which will typically mean that every directory on
        ``sys.path`` will be scanned for distributions.

        `installer` is a standard installer callback as used by the
        ``resolve()`` method. The `fallback` flag indicates whether we should
        attempt to resolve older versions of a plugin if the newest version
        cannot be resolved.

        This method returns a 2-tuple: (`distributions`, `error_info`), where
        `distributions` is a list of the distributions found in `plugin_env`
        that were loadable, along with any other distributions that are needed
        to resolve their dependencies.  `error_info` is a dictionary mapping
        unloadable plugin distributions to an exception instance describing the
        error that occurred. Usually this will be a ``DistributionNotFound`` or
        ``VersionConflict`` instance.
        N)
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