Produces the format that Nix calls “makeValidityRegistration” and describes as:
“A string accepted by decodeValidPathInfo() that registers the specified paths as valid.”
Explained differently, this is the text format used in exportReferencesGraph,
in the non-structured attrs case, which can be passed to nix-store --register-validity.
We don’t implement the showDerivers and showHash functionality,
as that doesn’t seem to be used for exportReferenceGraph.