Control join directive usage from the directive definition spec

[tninesling]

The primary issue with controlling the use of join directives via identity strings is that any logic relying on the presence of @join__directive won't work if the affected directive is imported from the federation spec.

More concretely, @link(url: "https://specs.apollo.dev/source/v0.1", import: ["@sourceAPI"]) results in a @join__directive for each use of @sourceAPI. In contrast, @link(url: "https://specs.apollo.dev/federation/v2.8", import: ["@sourceAPI"]) does not.

To make the second case work, we don't determine join usage by the URL used in the @link. Instead, each directive definition spec has a usesJoinDirective flag which determines this behavior. When we handle @link usages, composition will use a @join__directive to capture that link if any of its imports have the usesJoinDirective flag set. The resulting behavior change is that the supergraph SDL will have a net-new @join__directive application if any of the @source* directives are imported from the federation spec instead of the source spec.