Move GraphQL schema files to graphql area

[buskamuza]

GraphQL schema files make sense only in graphql area of Magento application.

AC:

• all schema.graphqls files are moved under graphql area folder
• validate that application behaves as expected in case a module (for example, an extension) that extends core GraphQL schema is still located in global area
• Ensure GraphQl style sniffs work properly after the change MC-19366: Adds GraphQL sniffs magento-coding-standard#141

This change should not introduce any breaking changes as files from global and graphql areas should be collected and merged during a GraphQL request. Extensions and customizations that use old convention and have GraphQL schema files located in global area should continue working as before.

[antonkril]

Don't think we should do it now:

• BC violation
• in an instance that only serves Graphql and only has Graphql modules installed, there is no benefit of Graphql area

[Vinai]

This would be a big BC break providing very little benefit from my point of view. I agree with @antonkril that they should remain where they are.