*: move "discovery" to "internal/discovery"

[gyuho]

For #9220.

[codecov-io]

Codecov Report

Merging #9233 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9233      +/-   ##
==========================================
+ Coverage   75.78%   75.92%   +0.14%     
==========================================
  Files         363      363              
  Lines       30190    30190              
==========================================
+ Hits        22880    22923      +43     
+ Misses       5708     5670      -38     
+ Partials     1602     1597       -5

Impacted Files	Coverage Δ
etcdmain/etcd.go	50.43% <ø> (ø)	⬆️
etcdserver/server.go	79.51% <ø> (ø)	⬆️
internal/discovery/discovery.go	74.03% <ø> (ø)
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
pkg/netutil/netutil.go	66.66% <0%> (-2.09%)	⬇️
clientv3/watch.go	95.58% <0%> (-0.25%)	⬇️
clientv3/namespace/watch.go	93.93% <0%> (ø)	⬆️
clientv3/health_balancer.go	85.97% <0%> (ø)	⬆️
etcdserver/v3_server.go	81.65% <0%> (+0.51%)	⬆️
... and 8 more

---

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[xiang90]

not sure about this one. discovery is a client side thing, not sure if people depend on it today.

[gyuho]

It's only used in etcdmain and etcdserver? GitHub and Google search results show no use of this package outside of etcd.

[xiang90]

if it is the case, then lgtm