Build dependencies before triggered components

[itowlson]

It's useful to be able to have dependencies built as part of spin build. However, for type information to flow correctly (e.g. in the presence of a Rust build script or a JS witDependencies-like section), it's desirable to build those dependencies before we build the primary components.

(To be clear, this is infrastructural work to support other intended tooling: it won't necessarily make a difference today, but it goes some way to enabling stuff that will make a difference tomorrow.)

cc @tschneidereit