[validator] Implement "Fragment Name Uniqueness".

[codecov[bot]]

Codecov Report

Merging #184 into master will increase coverage by 0.01%. The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   81.41%   81.43%   +0.01%     
==========================================
  Files         104      105       +1     
  Lines        9084     9135      +51     
==========================================
+ Hits         7396     7439      +43     
- Misses       1381     1385       +4     
- Partials      307      311       +4

Impacted Files	Coverage Δ
...raphql/validator/rules/lone_anonymous_operation.go	100% <ø> (ø)	:arrow_up:
...phql/validator/rules/single_field_subscriptions.go	100% <ø> (ø)	:arrow_up:
graphql/validator/rules/fields_on_correct_type.go	97.05% <ø> (ø)	:arrow_up:
graphql/validator/rules/init.go	100% <100%> (ø)	:arrow_up:
graphql/validator/rules/unique_operation_names.go	100% <100%> (ø)	:arrow_up:
graphql/internal/validator/messages.go	100% <100%> (ø)	:arrow_up:
graphql/validator/rules/unique_fragment_names.go	100% <100%> (ø)
graphql/validator/validation_context.go	93.1% <100%> (+0.24%)	:arrow_up:
graphql/validator/walk.go	92.72% <82.35%> (-4.27%)	:arrow_down:
... and 1 more

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