Validate schema documents before merging type extensions

[BoD]

type Query {
  nullableField: String
}

extend schema @link(
    url: "https://specs.apollo.dev/nullability/v0.3",
    import: ["@catch", "CatchTo", "@semanticNonNullField"] # not importing @semanticNonNull here
)
extend schema @catch(to: THROW)

extend type Query @semanticNonNullField(name: "nullableField")

In ExtensionsMerger.mergeObjectOrInterfaceDirectives, @semanticNonNull is implicitly added to the field's type. Validation fails later when it's not imported.

[martinbonnin]

Good catch 👍 We'll probably need to handle validation before the merging.