runtimevar/etcdvar: etcd has all sorts of go.mod problems

[vangent]

etcd-io/etcd#12124
etcd-io/etcd#11931
etcd-io/etcd#11749

This means we can't update etcd or a bunch of other unrelated modules (like cloud.google.com).

Plan is to drop support for etcd going forward until these are fixed. Users can continue using the current tagged version of go-cloud.

[saheljalal]

All the indicated issues appear closed now. Could we evaluate if support will be added back here?

[vangent]

I try it every once in a while:

$ cd runtimevar/etcd
$ go get -u
....
$ go build

I still get errors:

# github.com/coreos/etcd/clientv3/balancer/resolver/endpoint
../../../../../../pkg/mod/github.com/coreos/etcd@v3.3.27+incompatible/clientv3/balancer/resolver/endpoint/endpoint.go:114:87: undefined: resolver.BuildOption
../../../../../../pkg/mod/github.com/coreos/etcd@v3.3.27+incompatible/clientv3/balancer/resolver/endpoint/endpoint.go:182:40: undefined: resolver.ResolveNowOption
# github.com/coreos/etcd/clientv3/balancer/picker
../../../../../../pkg/mod/github.com/coreos/etcd@v3.3.27+incompatible/clientv3/balancer/picker/err.go:37:53: undefined: balancer.PickOptions
../../../../../../pkg/mod/github.com/coreos/etcd@v3.3.27+incompatible/clientv3/balancer/picker/roundrobin_balanced.go:55:63: undefined: balancer.PickOptions

[vangent]

I would love it if someone would tell me how to fix this, but I spent so much time on it a year ago with no success that I am totally unmotivated to do so now.