Migrate the `version` package from `github.com/prometheus/common`
to `github.com/prometheus/client_golang` in order to break the circular
dependency.
* Make `version` a top level package because it uses `init()` to
populate data.
Related to: https://github.com/prometheus/common/issues/58
Signed-off-by: SuperQ <superq@gmail.com>
SuperQ