resourceloader: Only store sources' load.php urls

Previously ResourceLoader would store any arbitrary data about a
source, provided it had a 'loadScript' key. It would register
the 'local' source with an additional 'apiScript' key, which was
also documented in DefaultSettings.php. However, it was
completely unused outside of the ForeignAPIGadgetRepo class in
Gadgets 2.0, which should be changed to take an API url as a
parameter. This was not useful as it was not ever formally
exposed, and it could not be depended upon that a source had
registered an 'apiScript' key.

For backwards compatability, both ResourceLoader::addSource()
and mw.loader.addSource() will both take an array/object, but
discard all parameters except for 'loadScript'.

Also added tests for ResourceLoader::addSource().

Bug: 69878
Change-Id: I4205cf788cddeec13b619be0c3576197dec1b8bf
Merge "resourceloader: Only store sources' load.php urls"