It turns out that it's possible for OSG plugins to be spread across multiple directories, and OSG doesn't account for this in osgDB::listAllAvailablePlugins(), even though it works when actually loading the plugin.
Instead, use code that's much more similar to how OSG actually loads plugin, and therefore less likely to miss anything.
Incidentally make things much simpler as we don't need awkwardness from working around osgDB::listAllAvailablePlugins()'s limitations.
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