I think this is actually done at runtime, so Umbraco is looking for classes of type PackageMigrationPlan so when your package is removed it also goes from the list.
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What happens if you remove the package? I guess it will still keep track of the initial migration, and keep saying that the package is installed?
I think this is actually done at runtime, so Umbraco is looking for classes of type
PackageMigrationPlanso when your package is removed it also goes from the list.