Support multi-module releases in go-control-plane.
From this point onward, the `go-control-plane` repository is released as multiple packages:
 - `github.com/envoyproxy/go-control-plane/envoy` and `github.com/envoyproxy/go-control-plane/contrib` are released with a version ~ matching  `github.com/envoyproxy/envoy` and should be imported to use generated go protobuf schemas
 - `github.com/envoyproxy/go-control-plane` is only versioning the `go-control-plane` core components (i.e. cache and server)
 - `github.com/envoyproxy/go-control-plane/ratelimit` and `github.com/envoyproxy/go-control-plane/xdsmatcher` are independent features versioned under their own tag

Signed-off-by: Alec Holmes <alecholmez@me.com>
Signed-off-by: Valerian Roche <valerian.roche@datadoghq.com>
Signed-off-by: Matthieu MOREL <matthieu.morel35@gmail.com>