[flakes] generate flake.nix file for php-related packages (#639)

## Summary

For `flake.nix`:
- we now also include the php "Definitions" and change the `buildInputs`
list so that it uses the `php` packages from `Definitions` instead of
from `nixpkgs` (if Definitions exist)

TODO for future PR:
- [ ] look into whether `buildInputs` should be `packages` in
`flake.nix`

## How was it tested?

in `examples/testdata/php/php-extensions`, started `devbox shell` with
`DEVBOX_FEATURE_FLAKES=0` and `DEVBOX_FEATURE_FLAKES=1`. Inspected the
generated `flake.nix` files.

```
❯ cat .devbox/gen/flake/flake.nix
{
  description = "A devbox shell";

  inputs = {
    nixpkgs.url = "https://github.com/nixos/nixpkgs/archive/f80ac848e3d6f0c12c52758c0f25c10c97ca3b62.tar.gz";

    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let pkgs = nixpkgs.legacyPackages.${system};
          php81 = pkgs.php81.withExtensions ({ enabled, all }: enabled ++ (with all; [ ds ]));
          php81Packages.composer = php81.packages.composer;

      in {
        devShell = pkgs.mkShell {
          shellHook =
            ''
              # We're technically no longer in a Nix shell after this hook because we
              # exec a devbox shell.
              export IN_NIX_SHELL=0
              export DEVBOX_SHELL_ENABLED=1

              # Undo the effects of "nix-shell --pure" on SSL certs.
              # See https://github.com/NixOS/nixpkgs/blob/dae204faa0243b4d0c0234a5f5f83a2549ecb5b7/pkgs/stdenv/generic/setup.sh#L677-L685
              if [ "$NIX_SSL_CERT_FILE" = "/no-cert-file.crt" ]; then
                 unset NIX_SSL_CERT_FILE
              fi
              if [ "$SSL_CERT_FILE" = "/no-cert-file.crt" ]; then
                 unset SSL_CERT_FILE
              fi

              # Append the parent shell's PATH so that we retain access to
              # non-Nix programs, while still preferring the Nix ones.
              export "PATH=$PATH:$PARENT_PATH"


            '';

          buildInputs = with pkgs; [
            php81

            php81Packages.composer

            php81Extensions.ds
            ];
        };
      }
    );
}
```

Author: savil

internal/impl/tmpl/flake.nix.tmpl

@@ -10,6 +10,9 @@
   outputs = { self, nixpkgs, flake-utils }:
     flake-utils.lib.eachDefaultSystem (system:
       let pkgs = nixpkgs.legacyPackages.${system};
+          {{- range .Definitions}}
+          {{.}}
+          {{- end }}
 
       in {
         devShell = pkgs.mkShell {
@@ -38,9 +41,9 @@
               {{- end }}
             '';
 
-          buildInputs = [
+          buildInputs = with pkgs; [
             {{- range .DevPackages}}
-              pkgs.{{.}}
+            {{.}}
             {{end -}}
           ];
         };