avoid O(num_modules) running time

[youngj]

there are a few parts of the code that are currently O(num_modules) which should be made O(1):

• engine/start.php includes start.php for each module. make.php should generate a file that concatenates each start.php file
• Engine::get_real_path() checks if the file exists in each module. make.php should make this O(1) by creating a path cache for each virtual directory
• Theme::load_all() checks every module for themes, even though most modules probably don't define themes

[youngj]

no longer necessary after recent module refactoring