Merge #18056: ci: Check for submodules

2a95c7c956 ci: Check for submodules (Emil Engler)

Pull request description:

  See #18019.
  The current solution looks like this (I also tested with multiple submodules):
  ```
  These submodules were found, delete them:
   355a5a310019659d9bf6818d2fd66fbb214dfed7 curl (curl-7_68_0-108-g355a5a310)
  ```
  The submodule example command was `git submodule add https://github.com/curl/curl.git curl`

ACKs for top commit:
  laanwj:
    ACK 2a95c7c956

Tree-SHA512: 64bf388123f0a88d12e3e41ff29bc190339377a0615c35dc3f2700bb7773470a8fa426e0ff57188a60ed88bded39f75082ff0b73118651ff403b163422395005

test/lint/lint-submodule.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# This script checks for git modules
+export LC_ALL=C
+EXIT_CODE=0
+
+CMD=$(git submodule status --recursive)
+if test -n "$CMD";
+then
+    echo These submodules were found, delete them:
+    echo "$CMD"
+    EXIT_CODE=1
+fi
+
+exit $EXIT_CODE
+