pay attention to package sources in export

[dimbleby]

previously the exporter would see two packages from different sources as the same, combine their markers, and produce nonsense like this:

foo @ https://example.com/foo-darwin.whl ; python_version >= "3.6" and python_version < "4.0" and sys_platform == "linux" or python_version >= "3.6" and python_version < "4.0" and sys_platform == "darwin"

[sonarqubecloud]

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