FindBoost: Revert "Simplify search in lists."

Revert commit v3.9.0-rc1~41^2 (FindBoost: Simplify search in lists,
2017-04-23).  It regressed the module by exposing issue #17257, but the
fix for that issue is not suitable for inclusion in a patch release.
It is simplest to revert the commit until the larger problem can be
addressed.

Fixes: #17252
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