Break-out the loader & indexer

[matthewmueller]

This stuff is separate from the actual translation and useful in really any program that transforms Go ASTs. I'm finding myself copying a lot of the logic for the React framework.

Maybe this should be in a separate repo or at least isolated and abstracted a bit.

[matthewmueller]

There's also improvements we can make that GopherJS already does around loading. They do custom archiving to speed up importing large packages like "fmt". We don't really have this problem yet, but having a standard place to put these Go files would be nice.

I think gopher's loading logic predates https://godoc.org/golang.org/x/tools/go/loader, so we'll probably want to use the standard but add in the archiving.