Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

When you have an independent theme/plugin from the WordPress repository, you expect WP-CLI to use the Update URI header if it is set.

For now, the wp plugin list command may show that an update is available event if the Update URI is set to false.

Describe the solution you'd like

When a plugin uses Update URI with a value of false or an URI that doesn't match the WordPress theme/plugin repositories, WP-CLI should handle that.

It should not be possible to update the theme/plugin when Update URI is set to false.
When there's an URI outside of the WordPress directory, it should use that URI.