dep_zapdeps: adjust || preference for slot upgrades (bug 706278)

Prefer choices that include a slot upgrade when appropriate, like for
the || ( llvm:10 ... llvm:7 ) case reported in bug 706278. In order
to avoid pulling in inappropriate slot upgrades, like those which
should only be pulled in with --update and --deep, add a want_update
flag to each choice which is True for choices that pull in a new slot
for which an update is desirable.

Mark the test case for bug 480736 as todo, since the "undesirable"
slot upgrade which triggers a blocker conflict in this test case is
practically indistinguishable from a desirable slot upgrade. This
particular blocker conflict is no longer relevant, since current
versions of media-libs/libpostproc are no longer compatible with
any available media-video/ffmpeg slot. In order to solve this test
case, some fancy backtracking (like for bug 382421) will be required.

Bug: https://bugs.gentoo.org/706278
Bug: https://bugs.gentoo.org/480736
Signed-off-by: Zac Medico <zmedico@gentoo.org>

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