**What type of PR is this?**
/kind cleanup
**What this PR does / why we need it**:
Staging the GCE Cloud Provider as part of KEP [20190125-removing-in-tree-providers](https://github.com/kubernetes/enhancements/blob/master/keps/sig-cloud-provider/20190125-removing-in-tree-providers.md). Staging repo setup here https://github.com/kubernetes/legacy-cloud-providers
Moves the GCE cloud provider implementation to staging.
This is in preparation for moving the cloud provider code out of tree entirely.
However we need it in staging while the code needs to be consumed both in/out of tree.
**Which issue(s) this PR fixes**:
Fixes #
**Special notes for your reviewer**:
**Does this PR introduce a user-facing change?**:
```
NONE
```
Updated import dependency tracking.
Factored in the cleanup from #77412
Minor fix to go.mod.
clusterCIDR is passed down from kube_env CLUSTER_IP_RANGE to the flag --cluster-cidr.
We plan not to let GKE to pass down CLUSTER_IP_RANGE to kube-env for the master node.
