Remove dependency on cli-runtime

[imjasonh]

Fixes #317

This change effectively forks the parts of genericclioptions that we use (i.e., global kubectl flag definitions) into an internal package, where flags are documented as deprecated. It severs our dependency on cli-runtime, and promotes an indirect dep on client-go to a direct one, for now. In a future change, we can remove this package too.

$ go run ./ create -f test --namespace=foo
2021/08/09 21:16:06 NOTICE!
-----------------------------------------------------------------
Passing kubectl global flags to ko directly is deprecated.

Instead of passing:
    ko create ... --namespace=foo

Pass kubectl global flags separated by "--":
    ko create ... -- --namespace=foo

For more information see:
    https://github.com/google/ko/issues/317
-----------------------------------------------------------------

If flags are passed before and after --, the one after the -- is used:

$ go run ./ create -f test --namespace=foo -- --namespace=bar
...
Error from server (NotFound): error when creating "STDIN": namespaces "bar" not found

Flags after -- aren't parsed at all by ko, and are just passed directly to kubectl:

$ go run ./ create -f test -- --unknown=flag
...
Error: unknown flag: --unknown
See 'kubectl apply --help' for usage.