"Alternate FOD" for fixed output derivation that fails, mirror-like behavior

[roberth]

Is your feature request related to a problem? Please describe.

Sometimes a fixed output derivation doesn't work. For example, a site may have gone offline.
However, the same content may be available in a different location, such as a mirror, web archive, or a peer to peer content addressable store such as IPFS.
Currently, only traditional mirrors lists can be used in anticipation of such failures.

Describe the solutions you'd like

"Inductive":

• Add a special field such as __alternateDerivation
• Only allow it when the output(s) are fixed
• Do not treat it as a dependency
• If the build fails, try the __alternateDerivation instead. It must produce an output with the same name and hash.

"Algebraic":

• Add a new derivation system value, such as builtin:alternatives
• Disallow the builder attribute.
• Require fixed outputs
• Instead only allow an alternatives list, referencing derivations that implement the fixed output derivation

Comparison

It feels more natural to have metadata to be specified in the latter "algebraic" design, although nothing stops us from doing it with the inductive syntax.
Uses of metadata:

• scheduling can deprioritize bad sources (e.g. domains that are down, protocols that don't work)
• users can configure to disallow certain protocols, etc

Additional context

Priorities

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