Update module path.
Created by: bglimepoint
The Caddy import path seems to have changed as per https://github.com/caddyserver/caddy/commit/f5720fecd663f521d832c1bca69e52ece43dc2b1
Building fails with the following error with the old module path.
go: github.com/mholt/caddy@v1.0.1: parsing go.mod: unexpected module path "github.com/caddyserver/caddy"
go: error loading module requirements
make: *** [Makefile:17: coredns] Error 1