Convert checkout to a regular action (#70)

__test__/verify-submodules-not-checked-out.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+if [ ! -f "./submodules-not-checked-out/regular-file.txt" ]; then
+    echo "Expected regular file does not exist"
+    exit 1
+fi
+
+if [ -f "./submodules-not-checked-out/submodule-level-1/submodule-file.txt" ]; then
+    echo "Unexpected submodule file exists"
+    exit 1
+fi