`kpt alpha rpkg copy` fails when package's upstream repository is unregistered

[ChristopherFry]

Expected behavior

You should always be able to create a new revision of a package using kpt alpha rpkg copy even if the package's upstream repository is unregistered.

Actual behavior

kpt alpha rpkg copy fails with Error: Internal error occurred: failed to fetch package revision ... cannot find repository ...

Information

Kpt and Porch are both on 1d3a99cb.

Steps to Reproduce

In Porch, register two repositories - the public kpt-samples repository repository and a second personal repository

1. Using kpt alpha rpkg clone, clone the echo/v1 package to your personal repository
2. Using kpt alpha rpkg propose and kpt alpha rpkg approve approve the package
3. Unregister the kpt-samples repostiory
4. Using kpt alpha rpkg copy, create a new revision of the package
5. Error occurs with above command

Error: Internal error occurred: failed to fetch package revision "kpt-samples-3cecbbbc68286534bf31a98b7ac7fa3de911f84a": cannot find repository default/kpt-samples: repositories.config.porch.kpt.dev "kpt-samples" not found