Return a list of advertised YANG module names with revisions.

        Avoid repeated modules.

def yang_modules(self):
        """
        Return a list of advertised YANG module names with revisions.

        Avoid repeated modules.
        """
        res = {}
        for c in self.capabilities:
            m = c.parameters.get("module")
            if m is None or m in res: continue
            res[m] = c.parameters.get("revision")
        return res.items()