Decouple topotools from vschema

[dbussink]

This removes the coupling between vschema and topotools just for a single helper function.

With this change, we don't end up pulling a large dependency tree for other utilities like schemadiff.

See also https://go-proverbs.github.io, "A little copying is better than a little dependency.".

Also see planetscale/schemadiff#22 (comment)

Related Issue(s)

Part of #14717

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• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 68.27%. Comparing base (248014e) to head (01170b9).

Files	Patch %	Lines
go/vt/vtgate/vindexes/vschema.go	40.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16008   +/-   ##
=======================================
  Coverage   68.27%   68.27%           
=======================================
  Files        1562     1562           
  Lines      197316   197319    +3     
=======================================
+ Hits       134708   134719   +11     
+ Misses      62608    62600    -8     

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[shlomi-noach]

Thank you