Though no match for its nifty title, the moonbeam farce that MTV-weaned director Julien Temple (Absolute Beginners) has concocted from retooled parts of E.T., Splash, Spaceballs and anything else handy, shapes up as an entertaining trifle. Forgive the airhead plot that hinges on a spaceship crash-landing in the swimming pool of a Valley-girl manicurist, played by Geena Davis. The fun comes from Temple's protean visual wit and the irresistible charm of Davis, who just won an Oscar for her role in…