dist: add CI job to detect files missing from distro

Also:
- delete previous, cmake-specific solution.
- move a CI script under `.github`.

Follow-up to a118a6ecdd #14323
Closes #14463

.github/scripts/distfiles.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+# Compare git repo files with tarball files and report a mismatch
+# after excluding exceptions.
+
+set -eu
+
+gitonly=".git*
+^.*
+^appveyor.*
+^buildconf
+^GIT-INFO.md
+^README.md
+^renovate.json
+^REUSE.toml
+^SECURITY.md
+^LICENSES/*
+^docs/examples/adddocsref.pl
+^docs/THANKS-filter
+^projects/Windows/*
+^scripts/ciconfig.pl
+^scripts/cijobs.pl
+^scripts/contributors.sh
+^scripts/contrithanks.sh
+^scripts/delta
+^scripts/installcheck.sh
+^scripts/release-notes.pl
+^scripts/singleuse.pl
+^src/tool_hugehelp.c.cvs
+^tests/CI.md"
+
+tarfiles="$(mktemp)"
+gitfiles="$(mktemp)"
+
+tar -tf "$1" \
+  | sed -E 's|^[^/]+/||g' \
+  | grep -v -E '(/|^)$' \
+  | sort > "${tarfiles}"
+
+git -C "${2:-.}" ls-files \
+  | grep -v -E "($(printf '%s' "${gitonly}" | tr $'\n' '|' | sed -e 's|\.|\\.|g' -e 's|\*|.+|g'))$" \
+  | sort > "${gitfiles}"
+
+dif="$(diff -u "${tarfiles}" "${gitfiles}" | tail -n +3 || true)"
+
+rm -rf "${tarfiles:?}" "${gitfiles:?}"
+
+echo 'Only in tarball:'
+echo "${dif}" | grep '^-' || true
+echo
+
+echo 'Missing from tarball:'
+if echo "${dif}" | grep '^+'; then
+  exit 1
+fi