Currently haskell.nix uses IFDs internally in a number of places. This prevents the use of `--option allow-import-from-derivation false` even when `materialized` nix is provided.
* Changes `materialize` function so that it returns the `materialized` path when possible instead of a derivation (updates import-and-filter-project.nix to cope with that).
* Materializes the spdx json file.
* Prevents haskell.nix looking for `cabal.project`, `cabal.project.local` and `cabal.project.freeze` files in hackage packages and the ghc-extra-projects.
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