Declare library_info in yml file

I propose that we move everything in hook_library_info() in a MYMODULE.library.yml or MYMODULE.assets.yml file.

It'll make it easier on themer to declare their script and CSS dependencies #1969916-5: Remove drupal_add_js/css from seven.theme and it's more likely they'll do it instead of some ugly PHP to implement in some file.

We'd keep library_info_alter() because that's very much needed. yml file would work because the information in the library hook isn't dynamic.