Moves 'gn gen' into setup.py Make tools/build.py more ergonomic.
* Make sync_third_party work in general * Un-break build.py and run_hooks.py on windows * Partially fix format.py on windows * Reduce code duplication between run_hooks and sync_third_party
To allow better tab completion for ./tools/build.py mv build_third_party.py sync_third_party.py