BuildStream fails to build if there are multiple link dependencies with same target

[nanonyme]

It was noticed through https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/tree/nanonyme/repro?ref_type=heads that when any element has multiple dependencies that are links with same target, the element will silently not build and it will not be checkoutable since it has not been built.

Deduplication happens correctly with bst show for build-depends, runtime-depends and depends. Full cache key is also identical with one link element and three link elements with same target.

[nanonyme]

Example output

===============================================================================
[--:--:--][aa497071][    pull:components/bpf.bst            ] START   freedesktop-sdk/components-bpf/aa497071-pull.20250315-214036.log
[--:--:--][aa497071][    pull:components/bpf.bst            ] STATUS  Pulling artifact aa497071 <- https://cache.freedesktop-sdk.io:11001
[--:--:--][aa497071][    pull:components/bpf.bst            ] INFO    Remote (https://cache.freedesktop-sdk.io:11001) does not have artifact aa497071 cached
[00:00:00][aa497071][    pull:components/bpf.bst            ] SKIPPED Pull
[00:00:27][        ][    main:core activity                 ] SUCCESS Build

Pipeline Summary
    Total:       107
    Session:     107
    Pull Queue:  processed 0, skipped 56, failed 0
    Fetch Queue: processed 0, skipped 56, failed 0
    Build Queue: processed 0, skipped 55, failed 0

This is in multiple levels wrong. Even if BuildStream fails to build something, it needs to notice it failed to build it and list it in failed builds.