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.
Credential provider is useful without the v1 API, move the only
dependency out so that we can more easily move credential provider to a
utility library in the future (other callers besides Kubelet may need to
load pull secrets like Docker).
This allows runtimes in different packages (dockertools, rkt, kuberuntime) to
share the same logic. Before this change, only dockertools support this
feature. Now all three packages support image pull throttling.