Some fetcher functions support SRI-style `hash` attribute in addition to legacy type-specific attributes. When `hash` is used `outputHashAlgo` is null so let’s complain when SRI-style hash value was not detected.
Such attributes match the form ${type}${separator}${hash}: True SRI uses dash as a separator and only supports base64, whereas Nix’s SRI-style format uses a colon and supports all the same encodings like regular hashes (16/32/64).
To keep this program reasonably simple, we will upgrade Nix’s SRI-like format to pure SRI instead of preserving it.
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