*: move "auth" to "internal/auth"

[mitake]

/cc @gyuho

For #9220.

[gyuho]

lgtm. thanks!

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ce0ad37). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #9243   +/-   ##
=========================================
  Coverage          ?   75.86%           
=========================================
  Files             ?      363           
  Lines             ?    30194           
  Branches          ?        0           
=========================================
  Hits              ?    22907           
  Misses            ?     5681           
  Partials          ?     1606

Impacted Files	Coverage Δ
internal/auth/range_perm_cache.go	66.17% <ø> (ø)
internal/auth/simple_token.go	87.03% <ø> (ø)
etcdserver/api/v3rpc/util.go	78.26% <ø> (ø)
etcdserver/apply.go	91.16% <ø> (ø)
etcdserver/apply_auth.go	78.62% <ø> (ø)
etcdserver/v3_server.go	81.65% <ø> (ø)
internal/auth/store.go	82.11% <ø> (ø)
clientv3/auth.go	95.94% <ø> (ø)
internal/auth/jwt.go	82.43% <ø> (ø)
etcdserver/api/v3rpc/watch.go	83.26% <ø> (ø)
... and 2 more

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