chore: Auto wire crates with binaries

nix/modules/crate.nix

@@ -20,14 +20,27 @@
       type = lib.types.attrsOf lib.types.raw;
       default = builtins.fromTOML (builtins.readFile ("${config.path}/Cargo.toml"));
     };
+    hasBinaries = lib.mkOption {
+      type = lib.types.bool;
+      readOnly = true;
+      description = ''
+        Whether the crate has binaries or not.
+
+        See <https://doc.rust-lang.org/cargo/reference/cargo-targets.html#binaries>
+      '';
+      default =
+        lib.pathIsRegularFile "${config.path}/src/main.rs" ||
+        lib.pathIsDirectory "${config.path}/src/bin" ||
+        lib.hasAttr "bin" config.cargoToml;
+    };
     autoWire = lib.mkOption {
       type = lib.types.bool;
-      default = false;
+      default = config.hasBinaries;
+      defaultText = "true if the crate has binaries, false otherwise";
       description = ''
         Autowire the packages and checks for this crate on to the flake output.
 
-        Generally, not all workspace creates need to be wired, only the ones
-        that the user actually uses need to be.
+        By default, crates with binaries will have their packages and checks wired.
       '';
     };
     crane = {