cmd/go: reject module paths containing '@' during 'go mod init'

[bcmills]

Extracted from #28338.

The module path in a go.mod file does not contain the version, but users (especially newcomers) may try to go mod init [email protected] or go mod init foo@latest anyway.

We should detect that and fail with a helpful error message, just as we reject pkg@version syntax in other commands (like go build).

[gopherbot]

Change https://golang.org/cl/150018 mentions this issue: cmd/go: disallow version string in go mod init module path

[DenWav]

Working link to CL: https://go-review.googlesource.com/c/go/+/150018