fizzy-spectests doesn't use import resolve API

[gumb0]

resolve_imported_functions and other resolve_* functions from #637 in their current form are not convenient to use to link exports/imports of several modules together. Resolve API and suits more to import host functions (because of convenience of flattened ImportedFunction struct and other Imported* - fewer curly braces on the caller side).

Importing from one module to another requires External* structs - which are returned from find_exported_* functions - that's what spectest runner uses.

Perhaps resolve API could be improved to accomodate both needs.