Restore warning for cookbook shadowing
Tidy cookbook upload specs
Add regression test for cookbook overlay warning
Add changelog note about cookbook overlay warning
Merge branch 'restore-shadowing-warning'

Commit cf8af09a40e5fd9fff8d1c481fb98b017545f49b removed warnings related to the
cookbook overlay/shadowing feature, so they have been missing since Chef 11.0.
Add them back.