After gathering more community feedback, more non-trivial use cases for
overriding `__findFile` emerged. Unlike the use case of Tvix mentioned
in #599, these can't easily be worked around by overriding `nixPath`
instead.
This is the second fixup/partial revert for
81d5f0a7d9.
See also #599.
Change-Id: I7ca75e1a2b196c0da341969c61f4c168b5f657f9