*: move "snap" to "internal/raftsnap"

[gyuho]

Fix #9207.
For #9220.

[codecov-io]

Codecov Report

Merging #9211 into master will decrease coverage by <.01%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master    #9211      +/-   ##
==========================================
- Coverage   76.06%   76.05%   -0.01%     
==========================================
  Files         363      363              
  Lines       30161    30161              
==========================================
- Hits        22941    22938       -3     
- Misses       5625     5631       +6     
+ Partials     1595     1592       -3

Impacted Files	Coverage Δ
raftsnap/db.go	63.33% <ø> (ø)
raftsnap/metrics.go	100% <ø> (ø)
raftsnap/snapshotter.go	72.04% <ø> (ø)
raftsnap/message.go	100% <ø> (ø)
etcdserver/storage.go	57.89% <100%> (ø)	⬆️
etcdserver/snapshot_merge.go	80% <100%> (ø)	⬆️
rafthttp/http.go	79.62% <100%> (ø)	⬆️
snapshot/v3_snapshot.go	62.96% <100%> (ø)	⬆️
rafthttp/snapshot_sender.go	91.04% <100%> (ø)	⬆️
rafthttp/peer.go	91.72% <100%> (ø)	⬆️
... and 27 more

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[xiang90]

lgtm. maybe we also need to add doc.go into raftsnap pkg to clarify that it is used by etcd internally for raft snapshot. (or it might be the time for use to move all internal pkg to /internal :P)

[gyuho]

@xiang90 Added doc.go and moved to internal with a separate issue to track other packages #9220.