Super Dev Mode: expose output module and sourcemap locations

Adds to the public API:

  JobEvent.getOutputModuleName()
  JobEvent.Builder.setOutputModuleName(String)
  CompileDir.findSourceMapFiles(String)

This is to make it easy to find the sizes of the sourcemap files
from outside the code server.

(Doesn't need to be in 2.7.)

Change-Id: Ic5b232a35b2ae13a3d2e0a40ab2777ba80521932
Review-Link: https://gwt-review.googlesource.com/#/c/9720/