*: move packages to "internal"

[gyuho]

For #9220.

@xiang90 How about auth package? It's used in clientv3 but indirectly

https://github.com/coreos/etcd/blob/ca14da00db7c9385b26ed8704eaff00edf413eb8/clientv3/auth.go#L46

[xiang90]

lgtm.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ca14da0). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #9238   +/-   ##
=========================================
  Coverage          ?   75.87%           
=========================================
  Files             ?      363           
  Lines             ?    30190           
  Branches          ?        0           
=========================================
  Hits              ?    22907           
  Misses            ?     5684           
  Partials          ?     1599

Impacted Files	Coverage Δ
clientv3/leasing/kv.go	91.36% <ø> (ø)
internal/mvcc/watcher.go	100% <ø> (ø)
auth/store.go	82.48% <ø> (ø)
pkg/mock/mockstore/store_recorder.go	89.65% <ø> (ø)
etcdserver/apply_auth.go	78.62% <ø> (ø)
internal/compactor/revision.go	69.04% <ø> (ø)
internal/mvcc/key_index.go	73.02% <ø> (ø)
etcdserver/api/v3rpc/util.go	78.26% <ø> (ø)
internal/mvcc/kvstore.go	90.87% <ø> (ø)
etcdctl/ctlv3/command/util.go	39.39% <ø> (ø)
... and 62 more

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