stack-to-nix/lts-to-nix doesn't understand snapshot extensions

[locallycompact]

I was trying to use stack-to-nix to print nix expressions for a stack snapshot extension however I've encountered two problems.

The first is that stack-to-nix does not understand urls in the resolver. Where

resolver: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2021/8/5.yaml

we have

AesonException "Error in $.resolver: expected String, but encountered Object"

The second problem is that stack-to-nix does not understand archive: entries in the packages: section, only in the extra-deps section. A snapshot extension must use packages to list archives, not extra-deps.