extract_utils: target_file() and target_args() cleanup

* Strip target_args from target_file at callee instead of at caller
* WARNING! Changes (improperly documented) behavior of prefix_match()
  function, which is expected to not strip target_args(), and the root
  cause why stripping target_args was currently done at caller. Will be
  addressed in next patch.

Change-Id: I820d2350aa64ff41374809fcb22f812257132652
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>