hack: introduce validate/no-module

Moby is not a Go module; to prevent anyone from mistakenly trying to
convert it to one before we are ready, introduce a check (usable in CI
and locally) for a go.mod file.

This is preferable to trying to .gitignore the file as we can ensure
that a mistakenly created go.mod is surfaced by Git-based tooling and is
less likely to surprise a contributor.

Signed-off-by: Bjorn Neergaard <bneergaard@mirantis.com>
(cherry picked from commit 25c3421802)
Signed-off-by: Bjorn Neergaard <bneergaard@mirantis.com>

.gitignore

@@ -13,8 +13,6 @@ thumbs.db
 .bashrc
 .editorconfig
 
-# top-level go.mod is not meant to be checked in
-/go.mod
 # build artifacts
 bundles/
 cli/winresources/*/*.syso

hack/validate/no-module

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+#
+# Check that no one is trying to commit a go.mod.
+
+SCRIPTDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOTDIR="$(git -C "$SCRIPTDIR" rev-parse --show-toplevel)"
+
+if test -e "${ROOTDIR}/go.mod"; then
+	{
+		echo 'FAIL: go.mod found in repository root!'
+		echo
+		echo '      Moby is not a Go module; please delete go.mod and try again.'
+	} >&2
+	exit 1
+else
+	echo 'PASS: No go.mod found in repository root!'
+fi