This also incorporates the version string into the package name so
that incompatibile versions will fail to connect.

Arbitrary choices:
- The proto3 package name is runtime.v1alpha2. The proto compiler
  normally translates this to a go package of "runtime_v1alpha2", but
  I renamed it to "v1alpha2" for consistency with existing packages.
- kubelet/apis/cri is used as "internalapi". I left it alone and put the
  public "runtimeapi" in kubelet/apis/cri/runtime.