Gentoo's solution to "too fast, too slow" addresses every requirement
in this post, except for this one:
On Mon, 2019-10-28 at 10:08 -0400, Stephen Gallagher wrote:
Requirement: Packagers must be able to encode whether their output
artifacts are intended for use by other projects or if they are
effectively private to the alternative version. Packagers must also
have a way of finding this information out so they understand what
they can and cannot rely on as a dependency.
I think adding this one feature to what Gentoo has done would be easier
than the path that has been taken so far.