flake.nix: extract packages/devshells/modules into smaller files

packages/libkrun/default.nix

@@ -0,0 +1,19 @@
+{ libkrun, libkrunfw, rustPlatform, variant ? null, ... }:
+
+let
+  libkrunfw' = libkrunfw.override { inherit variant; };
+in
+(libkrun.override {
+  withBlk = true;
+  withGpu = true;
+  withSound = true;
+  withNet = true;
+  inherit variant;
+  libkrunfw = libkrunfw';
+}).overrideAttrs (old: {
+  src = ../../libkrun;
+  cargoDeps = rustPlatform.importCargoLock {
+    lockFile = ../../libkrun/Cargo.lock;
+  };
+  # mesonFlags = [ (lib.mesonOption "decoders" "gles,vulkan,composer") ]; # no magma(?)
+})