compiler: include transitive imports in the type descriptor list

compiler: include transitive imports in the type descriptor list

    In CL 179598, we were using Gogo::packages_, when compiling the
    main package, as the list of packages of which we need to
    register the type descriptors. This is not complete. It only
    includes main's direct import and one-level indirect imports. It
    does not include all the packages transitively imported.

    To fix that, we need to track all the transitive imports. We
    have almost already done that, for init functions. However, there
    may be packages that don't need init functions but do need to
    register type descriptors. For them, we add a dummy init function
    to its export data. So when we compile the main package we will
    see all the transitive imports. The dummy init functions are not
    real functions and are not called.

    Fixes golang/go#32901.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/184717

From-SVN: r273009