release: disallow aliases

Having multiple attributes point to the same derivation makes debugging
harder because you often cannot just grep the canonical attribute name.
It is even annoying when multiple aliases are chained.

Having Hydra build aliases is recognized as redundant
https://github.com/NixOS/nixpkgs/commit/2d0a7c4eeecd22f26eb37f6077a0397c32250375
so let’s do the same with their evaluation.

This commit will make Hydra ignore aliases when evaluating.

A nice benefit of this is that it will allow us to warn users when
an attribute is renamed, to assist them with early migration. Since
tracing messages during evaluation are not allowed because of Hydra,
we can currently only choose between having a silent alias and throwing.

This was last attempted in 2018 but ended up being reverted
because of widespread use of aliases that was not caught by CI:
https://github.com/NixOS/nixpkgs/commit/8c025c67d5a151055ef63d2b0d94921604ff0f62
CI has since been improved:
https://github.com/NixOS/ofborg/commit/cda5aa2ac77a70bb5660d8d5614a640aacbe7523

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