factor out nix code into new files via makeScope

This makes the Nix code a lot easier to reason about.

nix/pkgs/default/default.nix

@@ -0,0 +1,64 @@
+# Keep sorted
+{ craneLib
+, inputs
+, lib
+, pkgsBuildHost
+, rocksdb
+, rust
+, stdenv
+}:
+
+let
+  env = {
+    CONDUIT_VERSION_EXTRA = inputs.self.shortRev or inputs.self.dirtyShortRev;
+    ROCKSDB_INCLUDE_DIR = "${rocksdb}/include";
+    ROCKSDB_LIB_DIR = "${rocksdb}/lib";
+  }
+  //
+  (import ./cross-compilation-env.nix {
+    # Keep sorted
+    inherit
+      lib
+      pkgsBuildHost
+      rust
+      stdenv;
+  });
+in
+
+craneLib.buildPackage rec {
+  inherit
+    (craneLib.crateNameFromCargoToml {
+      cargoToml = "${inputs.self}/Cargo.toml";
+    })
+    pname
+    version;
+
+  src = let filter = inputs.nix-filter.lib; in filter {
+    root = inputs.self;
+
+    # Keep sorted
+    include = [
+      "Cargo.lock"
+      "Cargo.toml"
+      "src"
+    ];
+  };
+
+  # This is redundant with CI
+  doCheck = false;
+
+  nativeBuildInputs = [
+    # bindgen needs the build platform's libclang. Apparently due to "splicing
+    # weirdness", pkgs.rustPlatform.bindgenHook on its own doesn't quite do the
+    # right thing here.
+    pkgsBuildHost.rustPlatform.bindgenHook
+  ];
+
+  inherit env;
+
+  passthru = {
+    inherit env;
+  };
+
+  meta.mainProgram = pname;
+}