Remove convenience re-exports from the `flatbread` package

[tonyketcham]

I think for scalability, we should not force certain Flatbread packages to be included in a user's downloaded node_modules if they are not necessary.

It's slightly less convenient but more direct about where plugins are coming from.

The weight of unneeded dependencies is something that the Gatsby community is annoyed about and we should set an early precedent to avoid that issue.

[odama626]

I think this will fix build watch dependencies too