'Prepare installation of a package.
If the package is already installed and force is false nothing
is done. If recursive is true prerequisites are installed first.
Returns a list of packages (to be passed to install) and a list
of messages of any problems encountered.'

def prepareInstall(self, package, force=0, recursive=1):

    self._curtodo = []
    self._curmessages = []
    self._prepareInstall(package, force, recursive)
    rv = (self._curtodo, self._curmessages)
    self._curtodo = []
    self._curmessages = []
    return rv
