chore: convert gatsby portal dependencies into workspaces

[paul-soporan]

What's the problem this PR addresses?

@yarnpkg/gatsby depends on various packages from our monorepo using the portal: protocol.

This causes Yarn to treat them as external projects instead of workspaces, bypassing various workspace-related features including enforcing constraints.

In addition, gatsby-plugin-yarn-introspection depends on @yarnpkg/monorepo using the workspace: protocol, which is weird and unintended (and should be unsupported) given the fact that they are part of 2 different projects.

(Spoiler: I'm also working on a change that will make this behavior unsupported.)

While things work in practice, semantically it makes more sense to declare them as workspaces and use the workspace: protocol.

How did you fix it?

Turned them into workspaces.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.