Revamps the way the exporter tracks exported types and imported
    packages that need to be mentioned in the export data.
    
    The previous implementation wasn't properly handling the case where an
    exported non-inlinable function refers to an imported type whose
    method set includes an inlinable function whose body makes a call to a
    function in another package that's not directly used in the original
    package.
    
    This patch integrates together two existing traversal helper classes,
    "Collect_references_from_inline" and "Find_types_to_prepare" into a
    single helper "Collect_export_references", so as to have common/shared
    code that looks for indirectly imported packages.
    
    Fixes golang/go#32778
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183850

From-SVN: r272955