Expose our packages in flake packages output

Currently, we can use bscpkgs similarly to nixpkgs either through
the flake outputs or with import bscpkgs:

```nix
# currently supported:
bscpkgs.legacyPackages.x86_64-linux.hello
let pkgs = import bscpkgs { system = "x86_64-linux"; }; in pkgs.hello
```
The missing piece is nixpkgs.lib (not pkgs.lib, the system agnostic
one). The workaround is to do bscpkgs.inputs.nixpkgs.lib instead. We can
simplify this by forwarding the lib to our outputs.

This enables us to use bscpkgs as a drop-in
replacing the inputs to our flake from nixpkgs to bscpkgs.
(inputs.nixpkgs.url = "<*BSC*pkgs url>").


Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
Tested-by: Aleix Boné <abonerib@bsc.es>

flake.nix

@@ -3,15 +3,36 @@
 
   outputs = { self, nixpkgs, ...}:
   let
+    system = "x86_64-linux";
     pkgs = import nixpkgs {
+      inherit system;
       # For now we only support x86
-      system = "x86_64-linux";
       overlays = [ self.overlays.default ];
     };
   in
     {
       bscOverlay = import ./overlay.nix;
       overlays.default = self.bscOverlay;
-      legacyPackages.x86_64-linux = pkgs;
+
+      # propagate nixpkgs lib, so we can do bscpkgs.lib
+      inherit (nixpkgs) lib;
+
+      # full nixpkgs with our overlay applied
+      legacyPackages.${system} = pkgs;
+
+      # packages added by our overlay
+      packages.${system} =
+        let
+          inherit (builtins) attrNames removeAttrs;
+          inherit (nixpkgs.lib) flip filterAttrs getAttrs pipe isDerivation;
+        in
+          # extract the names of the packages from the overlay, then get the
+          # actual packages from the full package set with the overlay applied
+          pipe (self.overlays.default null { inherit (nixpkgs) lib; } ) [
+            attrNames
+            (flip getAttrs pkgs)
+            (flip removeAttrs [ "bsc" ])
+            (filterAttrs (_: isDerivation))
+          ];
     };
 }