I feel like the embedded FS issue is orthogonal to the need for a #embed/include_bytes! equivalent. In that your proposal requires that language change, and that language change is useful immediately, without the rest of your proposal. I'd start there.
Previously on these forums somebody from the Swift team has expressed distaste for the idea of doing this, IIRC because it means that the compiler needs to load an input file which isn't mentioned on the command-line, and therefore potentially untracked as a dependency. So your solution would need to navigate those waters first, before taking on whatever SPM and swift-build integration is required.