there's not a really good name for the concept you describe: "a set of source files that share some common metadata",

Agreed, and that speaks to the bigger thing that we're realizing we want to do, but don't yet know how, particularly in a way that we don't screw over current users: somehow replace the idea of targets for first-party code with this merging-of-metadata idea Targets seem to work well when describing third party requirements + artifacts you want to build. They map very nicely 1-1 with the thing you're describing. Targets get really clunky when describing first party code, though.