Findlib library with missing `requires` crashes

[sim642]

If angstrom is installed from opam, but not angstrom-unix, then Findlib lookups can fail with:

Fl_package_base.No_such_package("angstrom-unix", "required by `angstrom.unix'")

Even Findlib.package_ancestors [] "angstrom.unix" crashes, even though it doesn't have to look up dependencies of angstrom-unix. Worse yet, Findlib.package_deep_ancestors will fail if any transitive dependency is missing like that, so not even the existing parts will be returned.