Support compilation of Swift framework bundles

- [x] Create a Swift framework
- [ ] Fix crash when a `swiftapp` task depends on a framework task

Some options for resolving the crash:

1. Deprecate `swiftmodule`. Managing linker paths and import paths manually is a pain.
2. When a `swiftmodule` or `swiftapp` task depends on a framework task (i.e., whose name ends in `.framework`), treat this as a special case and don't add the framework dependency to the "linkable node" graph.

I'm currently leaning towards the first option. I've started to think about how distribution of code build by lightspeed would actually work, and managing search paths and install locations for headers, swiftmodules, etc. would be more trouble than it's worth I think. Installing framework bundles into system locations makes sense, I think, but installing libs and headers into system locations doesn't seem worth it to me.