Proposal: Build dependency from archive located at arbitrary URL

This is a proposal for a feature. We would like to see if this feature makes sense to the community before we come up with a PR.

## Use case

We ship a proprietary library. There is aversion in the company against making our GitHub repo public.

At the same time we would like to give customers a way to build our library from source code.

We would like Carthage to support fetching non-binary, source code dependencies from arbitrary URL.

We do realize that the source code would still be available to download and inspect from the URL. But for our purposes it is enough that source code is not visible through GitHub UI (please don’t ask).

## Proposed design

Instead of checking out source code from GitHub tag, Carthage would resolve a URL to an archive, download source code, and then build an '.xcframework'.

To assist in dependency version resolution we could employ similar mechanism which is in place today for binary releases (a JSON dictionary where keys are version numbers and value is a URL to an archive)

We would like to hear your comments on this. In case you find it reasonable we’ll try to come up with a PR.