chore(license): add GPL-3.0 SPDX header to Go sources (#2173)

The legacy shell tree carried a per-file license header; the Go rewrite
shipped without one. Add it to all files and enforce it going forward
via golangci-lint's goheader linter, so the notice can't be dropped.

Signed-off-by: Luca Di Maio <luca.dimaio1@gmail.com>

internal/cli/stop.go

@@ -1,3 +1,22 @@
+// SPDX-License-Identifier: GPL-3.0-only
+//
+// This file is part of the distrobox project:
+//    https://github.com/89luca89/distrobox
+//
+// Copyright (C) 2021 distrobox contributors
+//
+// distrobox is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License version 3
+// as published by the Free Software Foundation.
+//
+// distrobox is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with distrobox; if not, see <http://www.gnu.org/licenses/>.
+
 package cli
 
 import (