compiler: don't read known type, simplify Import::finalize_methods

    
    With the current export format, if we already know the type, we don't
    have to read and parse the definition.
    
    We only use the finalizer in Import::finalize_methods, so make it a
    local variable.  To match Finalize_methods::type, only put struct
    types into real_for_named.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/197700

From-SVN: r276188