_compute_abi_rebuild_info: fix bug #521990

Since self._dynamic_config._slot_operator_deps only contains deps for
packages added to the graph, it doesn't contain potentially relevant
deps of installed packages that have not been added to the graph.
Therefore, generate pseudo-deps for such installed packages, and use
those to generate the rebuild info.

X-Gentoo-Bug: 521990
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=521990