When a definition is redeclared, the original code would clobber the
value of `const2ModIdx` every time, meaning that a constant would be
attributed to a module which occurs later than the modules for constants
referencing this one. Preferring the original module ensures that these
module indexes are dependency-ordered. This originally came up as a bug
in `shake`, which assumes this property, see
[Zulip](https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/check.20for.20unused.20imports.20doesn't.20stop/near/449139309).
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