[cgo] unloadLibrary is never called

We would need to call unload library with the wrapper destructor, but I am a bit helpless how this works. @qmuntal: Is there a chance you can take a look at this?

Start by tracing the cgo wrapper lifecycle and locating the wrapper destructor and unloadLibrary entry points. Determine how destruction is handled and verify that the library is unloaded when the wrapper is released; the issue provides no named files or tests.

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