use lib.makeScope and files to organize packages

Some of the improvements here include:

* rocksdb can actually use jemalloc now instead of just pulling in a
  second rocksdb for no reason
* "complement-runtime" factored back out into shell file
* complement image no longer uses `mkDerivation` for `copyToRoot`
  because that's what `buildEnv` is for
* complement image no longer sets `SERVER_NAME`, complement already does
  that
* all packages were factored out into `callPackage`-able files for use
  with a custom `lib.makeScope pkgs.newScope`
* new version of `mkPackage` has options that are easier to use and
  override such as `features`

nix/pkgs/main/default.nix

@@ -0,0 +1,71 @@
+{ inputs
+
+# Dependencies
+, craneLib
+, lib
+, libiconv
+, pkgsBuildHost
+, rocksdb
+, rust
+, stdenv
+
+# Options
+, features ? []
+, profile ? "release"
+}:
+
+craneLib.buildPackage rec {
+  src = inputs.nix-filter {
+    root = inputs.self;
+    include = [
+      "src"
+      "Cargo.toml"
+      "Cargo.lock"
+    ];
+  };
+
+  # This is redundant with CI
+  doCheck = false;
+
+  env =
+    let
+      rocksdb' = rocksdb.override {
+        enableJemalloc = builtins.elem "jemalloc" features;
+      };
+    in
+    {
+      CARGO_PROFILE = profile;
+      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 {
+      inherit
+        lib
+        pkgsBuildHost
+        rust
+        stdenv;
+    });
+
+  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
+  ]
+  ++ lib.optionals stdenv.isDarwin [ libiconv ];
+
+  cargoExtraArgs = ""
+    + lib.optionalString
+      (features != [])
+      "--features " + (builtins.concatStringsSep "," features);
+
+  meta.mainProgram = (craneLib.crateNameFromCargoToml {
+    cargoToml = "${inputs.self}/Cargo.toml";
+  }).pname;
+
+  passthru = {
+    inherit env;
+  };
+}