In links to packages, consider srcset-type mechanism

In whatever mechanism we use to offer a package to a browser, we should think about allowing browsers to choose optimized packages for their situations. There could be a fully-general package that includes many sizes of images that'll appear optimally on any device, but also a small-screen version of the same package that omits the high-resolution images entirely. There could also be other axes than screen size, along the lines of https://github.com/spanicker/device-ram.

This introduces some complications around having multiple different resources signed for the same URL. Normally, we could resolve such conflicts by just using the latest such resource, but it's possible that's one optimized for smaller screens than the current device.

This is not a v1 consideration.

No files, tests, or implementation entry points are named. The issue is exploratory and explicitly excludes v1, so first confirm scope and the package-selection and signing design with maintainers before identifying an implementation path. Done criteria are not defined in the issue.