[Build/PackageGraph] Switch `BuildSubset.{product, target}` and `Modu…

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…lesGraph.{product, target}(...)` to use optional destination

This is mostly an NFC change. It makes more sense to default destination to "undefined" when applicable because it's not always possible to know intended destination based on user input.

Motivation:

This kind of design makes more sense for internal APIs, instead of having users to pass .destination even though it might not be always correct.

Modifications:

• BuildSubset.{product, target} and ModulesGraph.{product, target}(...) accept an optional destination that is defaulted to .none

Result:

Easier to understand and use APIs to request a build subset and product and module references from the modules graph.

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@swift-ci please test

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@swift-ci please test

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@swift-ci please test Windows platform