Replace the setmatrix package's use of golang-set with a small local
map-backed set implementation. The package already serializes access with
its own mutex, so the external dependency was not buying us much and made
this small utility harder to keep self-contained.
Removing it trims the dependency surface and avoids carrying an otherwise
single-purpose module for a narrow internal use case.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>