Fix/udpa dependency reference

[kai5263499]

Update the upda dependency reference to fix this error I was seeing when running the go get github.com/fullstorydev/grpcurl/... installation step

package github.com/fullstorydev/grpcurl/cmd/grpcurl
	imports github.com/cncf/udpa/go/udpa/core/v1: cannot find package "github.com/cncf/udpa/go/udpa/core/v1" in any of:
	/usr/local/go/src/github.com/cncf/udpa/go/udpa/core/v1 (from $GOROOT)
	/home/wes/go/src/github.com/cncf/udpa/go/udpa/core/v1 (from $GOPATH)

[jhump]

The only changes in here, other than changing a bunch of imports to your own fork (which I clearly will not be able to accept), seems to be the way you are trying to pin the dependency.

However, it should already be pinned, indirectly, by the grpc-go module.

I think the issue is more complicated and likely has to do with your current working directory when you run go get .... I think the Go tool can end up running in GOPATH mode, depending on whether or not you are in a workspace or not. And in GOPATH mode, it is not using the go.mod constraints.

An issue has already been filed with upstream as this is a breaking change they introduced a little over a week ago: cncf/udpa#42