feat(fabric-server): create fabric-server package

pkgs/fabric-server/generate-loader.nix

@@ -0,0 +1,37 @@
+{ lib, fetchurl, stdenv, unzip, zip, jre_headless }:
+
+let
+  lock = import ./lock.nix;
+  libraries = lib.forEach lock.libraries fetchurl;
+in
+stdenv.mkDerivation {
+  name = "fabric-server-launch.jar";
+  nativeBuildInputs = [ unzip zip jre_headless ];
+
+  libraries = libraries;
+
+  buildPhase = ''
+    for i in $libraries; do
+      unzip -o $i
+    done
+
+    cat > META-INF/MANIFEST.MF << EOF
+    Manifest-Version: 1.0
+    Main-Class: net.fabricmc.loader.impl.launch.server.FabricServerLauncher
+    Name: org/objectweb/asm/
+    Implementation-Version: 9.2
+    EOF
+
+    cat > fabric-server-launch.properties << EOF
+    launch.mainClass=net.fabricmc.loader.impl.launch.knot.KnotServer
+    EOF
+  '';
+
+  installPhase = ''
+    jar cmvf META-INF/MANIFEST.MF "server.jar" .
+    zip -d server.jar 'META-INF/*.SF' 'META-INF/*.RSA' 'META-INF/*.DSA'
+    cp server.jar "$out"
+  '';
+
+  phases = [ "buildPhase" "installPhase" ];
+}