    )
    # add plugins+libs to sys.path
    map(working_set.add, distributions)
    # display errors
    print('Could not load', errors)

The `plugin_env` should be an ``Environment`` instance that contains
only distributions 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.
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