nix/fossbeamer: cleanup expression

- import -> callPackage
- don't use complex filter src function

nix/overlay.nix

@@ -1,49 +1,31 @@
-self: pkgs: rec {
-  fossbeamer_crates = import ../Cargo.nix {
-    inherit pkgs;
-    nixpkgs = pkgs.path;
+_: pkgs: {
+  fossbeamer = (pkgs.callPackage ../Cargo.nix {
+    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
+      fossbeamer = prev: {
+        src = pkgs.lib.fileset.toSource rec {
+          root = prev.src.origSrc;
+          fileset = (pkgs.lib.fileset.intersection
+            (pkgs.lib.fileset.fromSource root) # We build our final fileset from the original src
+            (pkgs.lib.fileset.fileFilter (f: f.hasExt "rs") root)
+          );
+        };
 
-    defaultCrateOverrides =
-      let
-        lib = pkgs.lib;
-        # Filters the given source, only keeping files related to the build, preventing unnecessary rebuilds.
-        # Includes src in the root, all other .rs files, as well as Cargo.toml.
-        # Additional files to be included can be specified in extraFileset.
-        filterRustCrateSrc =
-          { root # The original src
-          , extraFileset ? null # Additional filesets to include (e.g. test fixtures)
-          }:
-          lib.fileset.toSource {
-            inherit root;
-            fileset = (lib.fileset.intersection
-              (lib.fileset.fromSource root) # We build our final fileset from the original src
-              (lib.fileset.unions ([
-                (root + "/src")
-                (lib.fileset.fileFilter (f: f.hasExt "rs") root)
-                # We assume that every Rust crate will at a minimum have .rs files and a Cargo.toml
-                (lib.fileset.fileFilter (f: f.name == "Cargo.toml") root)
-              ] ++ lib.optional (extraFileset != null) extraFileset)));
-          };
-      in
-      pkgs.defaultCrateOverrides // {
-        fossbeamer = prev: {
-          src = filterRustCrateSrc { root = prev.src.origSrc; };
+        nativeBuildInputs = [
+          pkgs.wrapGAppsHook
+        ];
 
-          nativeBuildInputs = [ pkgs.wrapGAppsHook ];
-          buildInputs = with pkgs; [
-            glib-networking
-          ] ++ (with gst_all_1; [
-            gstreamer
-            gst-plugins-base
-            gst-plugins-good
-            gst-plugins-bad
-            gst-libav
-          ]);
-        };
+        buildInputs = with pkgs; [
+          glib-networking
+        ] ++ (with gst_all_1; [
+          gstreamer
+          gst-plugins-base
+          gst-plugins-good
+          gst-plugins-bad
+          gst-libav
+        ]);
       };
-  };
-
-  fossbeamer = fossbeamer_crates.rootCrate.build.override {
+    };
+  }).rootCrate.build.override {
     runTests = true;
   };
 }