_dep_check_composite_db: fix bug #526160

The fixes various forms of buggy behavior involving virtuals handling
and EAPI 5 subslots. The code at fault is part of the support for
bug #141118 (virtuals lookahead). This code was written long before
subslots existed, and because of them it needs to be updated now.

Previously, the relevant virtuals code would only select one virtual
from each slot for lookahead. Now, it needs to select one from each
slot/subslot pair. The buggy forms of behavior fixed by this change
can be difficult to reproduce, since they only express themselves
under some rare circumstances.

In addition to the _dep_check_composite_db fixes, there is also a
related fix inside _expand_new_virtuals, which is required so that
_slot_operator_update_probe can reliably check whether or not a
particular input atom was selected. Without this change, the
_dep_check_composite_db changes will cause the test from commit
d3be49fe6827aa1974856dffe6d5a1aca80a7bed to fail in a way that is
very similar to the failure reported in bug #526160.

Fixes: d3be49fe6827 ("depgraph: fix bug #526160")
X-Gentoo-Bug: 526160
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=526160