GlobalOpt: Don't assume we see all accesses to a global if it is imported from another module.
GlobalOpt: Don't assume we see all accesses to a global if it is imported from another module.

Currently we don't import (= deserialize) from other modules but we will if we serialize the StdlibUnitTests module.
I didn't add an explicit test case, because it's not that easy. But this change will be tested when running the suite with serialized StdlibUnitTest.

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