lib/Rex/CLI.pm: display a warning when/if Rex::Template is loaded.

[monsieurp]

Fixes #1475.

This PR is an attempt to fix #1475.

Checklist

• [x] based on top of latest source code
• [ ] changelog entry included: we might want to add one.
• [ ] tests pass in CI: let's see.
• [x] git history is clean
• [ ] git commit messages are well-written