A host of improvements in preparation for the 'bdist' command:
  - added 'get_outputs()' method (all the other improvements were to support
    this addition)
  - made 'find_package_modules()' and 'find_modules()' return similar
    values (list of (package, module, module_filename) tuples)
  - factored 'find_all_modules()' out of 'get_source_files()' (needed
    by 'get_outputs()')
  - factored 'get_module_outfile()' out of 'build_module()' (also needed
    by 'get_outputs()')
  - various little tweaks, improvements, comment/doc updates