Add source filtering

This resolves #4 by excluding the git subdirectory when building as a
flake.

Test builds with `nix build path:.` before and after the change.

default.nix

@@ -1,8 +1,9 @@
 { buildDunePackage
-, lib
 , core
 , core_unix
 , findlib
+, lib
+, nix-filter
 , ocaml
 , ppx_yojson_conv
 , ppx_yojson_conv_lib
@@ -13,7 +14,19 @@
 buildDunePackage {
   pname = "flake_env";
   version = "0.1";
-  src = ./.;
+  src = nix-filter {
+    root = ./.;
+    include = [
+      "bin"
+      "lib"
+      ./dune-project
+      ./flake.nix
+      ./default.nix
+      ./flake.lock
+      ./flake_env.opam
+      ./direnvrc
+    ];
+  };
   duneVersion = "3";
   postPatch = ''
     substituteInPlace direnvrc --replace "@flake_env@" "$out/bin/flake_env"

flake.lock

@@ -20,6 +20,21 @@
         "type": "github"
       }
     },
+    "nix-filter": {
+      "locked": {
+        "lastModified": 1701697642,
+        "narHash": "sha256-L217WytWZHSY8GW9Gx1A64OnNctbuDbfslaTEofXXRw=",
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "rev": "c843418ecfd0344ecb85844b082ff5675e02c443",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1702272962,
@@ -39,6 +54,7 @@
     "root": {
       "inputs": {
         "flake-parts": "flake-parts",
+        "nix-filter": "nix-filter",
         "nixpkgs": "nixpkgs"
       }
     }

flake.nix

@@ -6,9 +6,12 @@
       url = "github:hercules-ci/flake-parts";
       inputs.nixpkgs-lib.follows = "nixpkgs";
     };
+    nix-filter = {
+      url = "github:numtide/nix-filter";
+    };
   };
 
-  outputs = inputs @ { flake-parts, ... }:
+  outputs = inputs @ { flake-parts, nix-filter, ... }:
     flake-parts.lib.mkFlake { inherit inputs; }
       ({ lib, ... }: {
         systems = [
@@ -20,7 +23,7 @@
         ];
         perSystem = { config, pkgs, self', ... }: {
           packages = {
-            flake_env = pkgs.ocamlPackages.callPackage ./default.nix { };
+            flake_env = pkgs.ocamlPackages.callPackage ./default.nix { inherit nix-filter; };
             default = config.packages.flake_env;
           };
           devShells.default = pkgs.mkShell {