Don't call 'set_final_options()' in 'run()' anymore -- that's now
guaranteed to be taken care of for us by the Distribution instance.
Rearranged to bit to allow outsiders (specifically, the 'dist' command)
to find out what modules we would build:
- 'find_modules()' renamed to 'find_package_modules()'
- most of 'build_modules()' abstracted out to 'find_modules()'
- added 'get_source_files()' (for the 'dist' command to use)
- drastically simplified 'build_modules()' -- now just a wrapper around
'find_modules()' and 'build_module()'
of the 'install_py' command rather than 'build_py'. Obviously, this
meant that the 'build_py' and 'install_py' modules had to change; less
obviously, so did 'install' and 'build', since these higher-level
commands must make options available to control the lower-level
commands, and some compilation-related options had to migrate with the
code.