Suggestions for python-pillow#9398

• Rearrange code
• Rename _load_plugin_for_extension() to _import_plugin_for_extension(). It might be better to call this 'lazy importing', rather than 'lazy loading', as one might refer to the fact that we don't load() images on open() as 'lazy loading'
• format overrides file extension in save(). If format is specified, then we don't need to import based on the extension
• Added logger messages when importing based on extension, to match init()