fix: use output when looking up relational related type index

[cjihrig]

Description of changes

This allows the implicitly added 'id' field to be properly used.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[SwaySway]

lgtm pending ci

[codecov-commenter]

Codecov Report

Merging #8657 (d61bddd) into master (70547ad) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8657   +/-   ##
=======================================
  Coverage   56.88%   56.88%           
=======================================
  Files         756      756           
  Lines       42238    42239    +1     
  Branches     8649     8649           
=======================================
+ Hits        24027    24028    +1     
  Misses      17390    17390           
  Partials      821      821           

Impacted Files	Coverage Δ
...mplify-graphql-relational-transformer/src/utils.ts	100.00% <100.00%> (ø)

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