cloud versioning: don't push cloud-versioned imports

[dberenbaum]

Related to #8274.

If using import-url with versioning enabled, it's generally wasteful to make a copy of the imported data on dvc push since the original source should continue to be accessible unless someone deletes that version of the file. DVC should have a way to avoid pushing the imported data in this scenario and always relying on the original source (the same could be true for a static data source that isn't cloud-versioned).

[dberenbaum]

Also related to #4527. We need a way to turn on/off pushing to a remote.