*: revert "internal" package changes

[gyuho]

External tools like "etcd-dump-db" outside of etcd repository need
access to "mvccpb" and other packages. Should not be under internal.

Also projects that use clientv3 use mvccpb.PUT, and so on.

See https://github.com/search?l=Go&q=mvccpb.&type=Code&utf8=%E2%9C%93.

[gyuho]

Marked it as WIP.

We need to research more if there's any better way to handle it, before 3.4 release.

[gyuho]

I will revert internal packages for now, since it's breaking a lot of external projects relying on authpb, mvccpb, etc. We can discuss again how to organize internal packages later.

[codecov-io]

Codecov Report

Merging #9274 into master will increase coverage by 0.01%.
The diff coverage is 50.99%.

@@            Coverage Diff             @@
##           master    #9274      +/-   ##
==========================================
+ Coverage   72.74%   72.76%   +0.01%     
==========================================
  Files         361      361              
  Lines       30717    30717              
==========================================
+ Hits        22346    22352       +6     
+ Misses       6766     6758       -8     
- Partials     1605     1607       +2

Impacted Files	Coverage Δ
etcdserver/v2store/watcher.go	86.36% <ø> (ø)
etcdserver/v2store/event.go	100% <ø> (ø)
mvcc/backend/config_linux.go	100% <ø> (ø)
etcdctl/ctlv2/command/backup_command.go	67.56% <ø> (ø)	⬆️
etcdserver/v2store/metrics.go	92% <ø> (ø)
mvcc/kv.go	40% <ø> (ø)
etcdserver/backend.go	70.58% <ø> (ø)	⬆️
etcdserver/corrupt.go	54.42% <ø> (ø)	⬆️
etcdserver/v2error/error.go	91.66% <ø> (ø)
compactor/compactor.go	0% <ø> (ø)
... and 111 more

---

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