fix: prevent @index/@primaryKey from referencing itself

[cjihrig]

Description of changes

This commit prevents the sortKeyFields on @index and @primaryKey from including the field the directive is defined on.

Description of how you validated changes

Manual and unit testing

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

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

[codecov-commenter]

Codecov Report

Merging #8561 (e61bb60) into master (02894dc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8561      +/-   ##
==========================================
+ Coverage   57.14%   57.15%   +0.01%     
==========================================
  Files         724      724              
  Lines       40845    40856      +11     
  Branches     8360     8361       +1     
==========================================
+ Hits        23342    23353      +11     
  Misses      16693    16693              
  Partials      810      810              

Impacted Files	Coverage Δ
...index-transformer/src/graphql-index-transformer.ts	100.00% <100.00%> (ø)
...transformer/src/graphql-primary-key-transformer.ts	100.00% <100.00%> (ø)
...ges/amplify-graphql-index-transformer/src/utils.ts	100.00% <100.00%> (ø)

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