SPM Support for Binaries Distribution

[hartbit (David Hart)]

@ddunbar

From a design space perspective, what a lot of it comes down to is whether a single package can be both source and binary. If that is allowed, it alters the design space for #2.

I think we can safely assume that targets can be either source or binary. In cases where library maintainers want to support both cases, they can always have a source target and a binary target (and associated source and binary products) and has some script build the source into the binary folder on each release.