Only local files should be passed to _locale_parse_js_file()

Hi, just want to let you know that after upgrading core to 8.6.15 and background_image to 8.x-1.3
it's not possible to explore admin/config/media/background_image/{background_image}/edit pages anymore.

This is caused by locale module, because it can parse local file only, so it can't parse file from 'https://cdn.jsdelivr.net/npm/jscolor-picker@2.0.4/jscolor.min.js'.

the solution appears to be quite simple, declaring the library to be external solves the issue, see attached patch