SL-957: Don't move viewer executable until final viewer_manifest run.

viewer_manifest is run twice by CMakeLists.txt: once with --actions=copy and
once with the default actions copy and package. The fact that we (try to) move
the Mac viewer executable both times has confused things on a number of
occasions. Currently it's bollixing our attempt to run dsymutil on the
executable so we can package up the Mac symbols for both codeticket and
BugSplat.

Introduce DarwinManifest.is_rearranging() that tests whether either "package"
or "unpacked" is in self.args["actions"], echoing an earlier test. Make
several things conditional on that, notably moving the executable.