Allow grammar for `package-repositories`

[lengau]

What needs to get done

Allow us to use grammar for at least the url field of package-repositories.

Why it needs to get done

If I want to enable deb-src and build a rock for both amd64 and arm64, I need something like this:

package-repositories:
  - type: apt
    formats: [deb, deb-src]
    components:
      - main
      - restricted
      - universe
      - multiverse
    suites:
      - questing
    url: http://archive.ubuntu.com/ubuntu
    key-id: F6ECB3762474EDA9D21B7022871920D1991BC93C
  - type: apt
    formats: [deb, deb-src]
    components:
      - main
      - restricted
      - universe
      - multiverse
    suites:
      - questing
    url: http://ports.ubuntu.com/
    key-id: F6ECB3762474EDA9D21B7022871920D1991BC93C

But this then causes issues with apt because the architecture isn't available.

It would be nice to be able to say:

package-repositories:
  - type: apt
    formats: [deb, deb-src]
    components:
      - main
      - restricted
      - universe
      - multiverse
    suites:
      - questing
    url: 
      - to amd64: http://archive.ubuntu.com/ubuntu
      - else: http://ports.ubuntu.com
    key-id: F6ECB3762474EDA9D21B7022871920D1991BC93C