By JOHN AUSTIN How’s a North Texas act supposed to know when Grey’s Anatomy is looking for a song or whether a Boston nightclub might offer an open tour date for a band that’s never been east of Arlington?Computers and the Internet, of course.A good many would-be professional musicians have learned to produce, distribute and promote their music themselves rather than relying on agents or labels to make the contacts that lead to gigs at clubs and festivals or licensing deals on TV and films.Take 22-year-old Fort Worth singer/songwriter Tim Halperin, who on Thursday played at a promotional concert in Dallas sponsored by the Gap clothing store chain.