extindex: maintain pack symlinks and use "git multi-pack-index"

This is a fair amount of complexity, but it speeds up
"git cat-file --batch" startup by 3-4% with 50K packfiles
with a hot kernel cache.

This appears extremely sensitive to RAM available to
the kernel page cache with my SATA 2 SSD.  Faster storage
and more RAM can bring loading pack.

2.60s vs 2.69s were the best cases on my workstation with and
without the multi-pack-index, however times could be all over
the place (even in the minutes) with more activity on my
workstation.

Getting sub-minute times requires a git patch to speed up
alt_odb_usable():
  <https://lore.kernel.org/20210624005806.12079-1-e@80x24.org/>
Otherwise, prepare to wait several minutes.

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