Clean up boot vs non-boot disambiguating types

Haddock PR: https://github.com/haskell/haddock/pull/1185

We often have (ModuleName, Bool) or (Module, Bool) pairs for "extended" module names (without or with a unit id) disambiguating boot and normal modules. I think this is important enough across the compiler that it deserves a new nominal product type.

With that written, I think the IsBoot enum type in GhcMake is overkill; just use the same product. I first tried to use both the custom product and enum everywhere---against my judgement but in order to make sure we never regressed in inference or boolean blindness---but the refactor was annoying so I so switched tacks.

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