fix go.etcd.io/etcd versioned import path

[philips]

There are other bugs related to this but I am trying to summarize the issue, history of why this was rolled back, and how to fix it.

Problem:

etcd is currently incompatible with the Go module versioning. This is an example using go1.14.2:

$ go mod init test-etcd-client/v3
$ go get go.etcd.io/etcd/clientv3@v3.4.7
go get go.etcd.io/etcd/clientv3@v3.4.7: go.etcd.io/etcd@v3.4.7: invalid version: module contains a go.mod file,
so major version must be compatible: should be 
v0 or v1, not v3

Why is this important?

• Go ecosystem developer workflows are broken (see above)
• https://pkg.go.dev/go.etcd.io/etcd?tab=doc doesn’t default to the latest release
• Go mod versioning features are defaulting to on in go1.14 which may cause additional when we move past go1.13 as our default; might as well figure it out now

What needs to be checked

• Is etcd using a version of go with go modules now? (Yes, go1.13)
• Update all docs to point to the new package
• ...?

Side effects that need investigation

• double check etcd-dump-logs issue Add /v3 suffix to go.mod and update import paths #11221

Unavoidable Side Effects

• This will make backporting fixes slightly annoying for awhile as I think the package version changes should only go into master. (See previously reverted change https://github.com/etcd-io/etcd/pull/10766/files)

Paths forward

• Remove go.mod - this will make Go’s versions incompatible with the wider Go community
• Reintroduce the versioned paths reverted in *: reverting versioned import paths #10766

[purpleidea]

Remove go.mod

Please do this. It's unhelpful to have go mod IMHO... vendor/ worked great before. I know I'm probably in the minority to say it, but go.mod has caused more problems than it solved. Happy to discuss more if it would help you.

[gyuho]

Last attempted here #11349. Proto gen was not working...

If anybody can help, it will be greatly appreciated!

[philips]

PR ready for merging, I think #11823