player/{core,loadfile}: make cover art loading more explicit

Now loading cover art through mp_add_external_file requires an
additional argument to be set to true. This way not all video-add
commands end up being marked as cover art when they move through
mp_add_external_file, as originally changed in 55d7f9ded197d82d172b7baf74b1a07640361ae8 .

Additionally, this lets us clean up some logic that would otherwise be
duplicated between open_external_files and autoload_external_files, if
the logic had been kept split from mp_add_external_file.

Fixes #8358

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