* Update devshell Update nixpkgs and simplify devshell * Run ormolu * Fixups * Format nix files * Update sources
The file can now be imported even if there is no sources.json in the same directory.
This updates the sources.nix to replace the `outPath` field of package `foo` with the content of envionment variable `NIV_OVERRIDE_foo`. In the environment variable name, all characters outside of `[a-zA-Z0-9_]` are escaped to `_`.