Check if imported module exists in MODULES.

def visit_Import(self, node):
        """ Check if imported module exists in MODULES. """
        for alias in node.names:
            current_module = MODULES
            # Recursive check for submodules
            for path in alias.name.split('.'):
                if path not in current_module:
                    raise PythranSyntaxError(
                        "Module '{0}' unknown.".format(alias.name),
                        node)
                else:
                    current_module = current_module[path]