plugin/etcd: upgrade to go.etcd.io/etc import path

[miekg]

etcd moved their import paths to go.etcd.io/etc. But if I try to use the new ones I get the following error:

plugin/etcd/etcd.go:80:22: cannot use r.Kvs (type []*"github.com/coreos/etcd/mvcc/mvccpb".KeyValue) as type []*"go.etcd.io/etcd/mvcc/mvccpb".KeyValue in argument to e.loopNodes

Some indirect dependencies is pulling down etcd from the older location and somehow this gets used when compiling this.

I tried playing around with replace and exclude in go.mod but no avail.
I also don't know how to track down which indirect dep is pulling it on the older name

[chrisohaver]

There is go mod why ... e.g. go mod why -m github.com/coreos/etcd which helps a little.
That says ...

# github.com/coreos/etcd
github.com/coredns/coredns/plugin/etcd
go.etcd.io/etcd/clientv3
github.com/coreos/etcd/auth/authpb

Digging into it, it looks that 3.3.13 still references the old path in some files in go.etcd.io/etcd/clientv3. It appears to be fixed in master.

[miekg]

etcd-io/etcd#10943 They aim for a release on August 30