Without this, even passing -Dgtk_doc=false will cause documentation to
be built if gi-docgen is available.

Fixes: 82119a6b ("gi-docgen: Initial commit")

We don't have guarantees that the children will be in a predictable order,
which can be annoying for tracking down issues users have that the
developer cannot reproduce. This just normalizes things so that we can be
more sure that the issues will be predictable.

If we have a plugin we've added that we could not resolve all the
dependencies for then push it to the tail of the queue. That way, further
insertions of plugins are more likely to sort before the plugin.

Otherwise, we can push the info to the head as it has no dependencies.

This improves a situation in Builder where plugins that dependend on
plugins which also had dependencies could end up loading in the wrong
order.

We realistically aren't running on these systems, but we might as well be
explicit with the comment so that people know what the comment was
referring to.

If we have unresolved plugin info, we might need to place the item sooner
than the tail as another plugin could depend on this one.