Multi-package upgrades support

[samuel-williams-da]

The upgrades field in the daml.yaml is not currently integrated with multi-package. Ideally it can be automatically built when needed as per data-dependencies. Also, Package name imports aims to remove explicit dar paths from the daml.yaml, so should implement its logic for the upgrades field. However, it is unreasonable to do this without the multi-build support, for consistency. Concretely, we should:

• Support multi-build for the upgrades field
• If implemented, extend Package name imports to support the upgrades field.

Feature work:

• right click support "Go to previous version m.n" / "Compare with version m.n". Should double check with product.

[samuel-williams-da]

Partially addressed by 19481

[paulbrauner-da]

Would also be nice: in daml assistant or daml studio, a way to "create a new version of a package" that would create a copy of the package with a new version number.