Do not include helper packages in GNOME closures

[jtojnar]

The wrapGAppsHook adds every interesting directory of every dependency to the path, even when the dependency is only needed during build. For example, the binaries of appstream-glib or gobjectIntrospection are often used during build but the resulting closure does not need to include them. We need to split out a bin output for such tools.

[stale]

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