Move "pull" and "push" to graph/pull.go and graph/push.go

This is part of the ongoing effort to remove the deprecated server/
package, and generally cleanup and simplify the codebase.

Signed-off-by: Solomon Hykes <solomon@docker.com>
Merge pull request #7496 from shykes/cleanup-push-pull

Cleanup: move "pull" and "push" to graph/pull.go and graph/push.go