Support static ES module build with external deps

[guybedford]

This use case is tricky because we can't just modify a single exports object like the dynamic formats, but need some way to outputting the correct named exports. This type of analysis is actually more like what is done in Rollup, so the implementation suggestion here is to initially only support this for ES-module-only SFX bundles which are fully-rollup managed. Then this would be pending #205.

[guybedford]

(NB this is from lib/compile.js SFX wrapping external dependencies)

[guybedford]

Released in 0.15.0.