This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
Printing the values in ctx.String() will data race if the value
is mutable and doesn't implement String(), which seems to be common.
Instead, just return a fixed string instead of computing it; v3client
watches don't need as much flexibility for creating separate strings,
so separate ctx strings probably aren't necessary at this point.
Fixes#7811
