Added 'package' option.

Added 'package' option.
Catch up with renamed 'platdir' -> 'build_platlib' option in 'build'.
Don't call 'set_final_options()' in 'run()' anymore -- that's now
  guaranteed to be taken care of for us by the Distribution instance.
If 'include_dirs' is a string, split it on os.pathsep (this is half-
  hearted -- support for setting compile/link options on the command
  line is totally lame and probably won't work at all).
Added 'get_source_files()' for use by 'dist' command.
Added code to 'build_extensions()' to figure out the "def file" to use
  with MSVC++ and add it to the linker command line as an "extra_postarg".