Refactor UnitInfo

* Rename InstalledPackageInfo into GenericUnitInfo

The name InstalledPackageInfo is only kept for alleged backward
compatibility reason in Cabal. ghc-boot has its own stripped down copy
of this datatype but it doesn't need to keep the name. Internally we
already use type aliases (UnitInfo in GHC, PackageCacheFormat in
ghc-pkg).

* Rename UnitInfo fields: add "unit" prefix and fix misleading names

* Add comments on every UnitInfo field

* Rename SourcePackageId into PackageId

"Package" already indicates that it's a "source package". Installed
package components are called units.

Update Haddock submodule

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