[lit] Move formats into their own subpackage.
[lit] Move executeCommand() into lit.util.
[lit] Factor ShTest format script command parsing from other processing.
[lit] Fix tests to execute lit with same python as invoked with.
[lit] Avoid StringIO.
 - We barely used it, and it is very hard to use in a 2.5-3 compatible
   way because of changing expectations for its input types.
[lit] Fix a relative import issue I missed earlier.