Limit uniqueness to `@skip`, `@include` and `@deprecated` directives

At a glance

• Identifier: #471
• Stage: RFCX: Closed 2020-01-10T22:41:32Z
• Champion: @OlegIlyenko
• PR: Limit uniqueness to `@skip`, `@include` and `@deprecated` directives

Timeline

• Commit pushed: Include `@deprecated` directive in the validation rule on 2018-06-25 by @OlegIlyenko
• Spec PR created on 2018-06-24 by OlegIlyenko
• Commit pushed: Limit uniqueness to `@skip` and `@include` directives on 2018-06-24 by @OlegIlyenko

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This proposal directly relates to a discussion in https://github.com/facebook/graphql/issues/429. As was discussed at the latest WG meeting, I'm creating several alternative proposals. This one implements proposed solution 3. Limit the validation to only @skip and @include directives.

It limits the scope of "Directives Are Unique Per Location" to @skip and @include directives only.

This proposal is mutually exclusive with other alternative proposals:

• [RFC] Limit uniqueness to @skip, @include and @deprecated directives (#471, this one)
• [RFC] Limit directive uniqueness to explicitly marked directives (#472)

@leebyron @IvanGoncharov @jjergus I would appreciate your reviews.

Closes #429