Improve messaging about unknown artifact refs

We usually use the ???????? key (cache key length filled in with question marks) to indicate artifact refs that are as yet unknown, and most of the time this makes sense (like before an element gets tracked).

When attempting to pull artifacts in non-strict mode however, it only half makes sense.

Looking at these messages:

[--:--:--][????????][ pull:core-deps/spice-gtk.bst       ] STATUS  Pulling artifact ???????? <- https://cache.gitlab.gnome.org:11001
[--:--:--][????????][ pull:core-deps/spice-gtk.bst       ] INFO    Remote (https://cache.gitlab.gnome.org:11001) does not have ???????? cached

What makes sense to me is the left hand [????????] portion of the log lines; BuildStream is basically saying that we dont yet know what the actual key will be - this is to say that even though we might have a local artifact which matches the weak key, we still try to pull a remote artifact and when we do that, we try to prefer the strong key.

What doesnt make sense here is "Pulling artifact ???????? ....", and "... does not have ???????? cached" - clearly we do know what artifact we are trying to pull, but we are blindly using the element's artifact display key in this message and this is confusing/misleading.