driver: Store the HomePackageTable in a mutable reference

This commit refactors the HomePackageTable and HomeUnitGraph:

(1) It fixes a quadratic-in-the-number-of-modules space leak in upsweep ( #25511 (closed))

(2) And it reworks these structures into their own modules to simplify the driver. The refactor is driven by the introduction of IO in the HPT interface, but is a bit more aggressive in simplifying the interfaces to enforce correct usage (ie to avoid performance pitfalls).

Specifically:

• The HomeUnitGraph (HUG) is now in GHC.Unit.Home.Graph
• The HomePackageTable (HPT) is now in GHC.Unit.Home.PackageTable
  • The HPT now stores an IORef with the table of loaded home package modules.
  • The interface to the HPT now requires IO
  • The interface now enforces that the HPT is a datastructure that only grows
  • This is not enforced in the interface, but, clients of the HPT should never care about there being more or less entries in the HPT when these additional entries are not relevant to their result.
  • The exception to the invariant that the HPT is monotonically increasing is restrictHpt, a function which is called at a "barrier point" (during which there are no other threads inspecting or inserting in the HPT). The invariant can be temporarily broken at this point (currently, after upsweep). This is safe because a single thread holds control over the structure (thus the invariant being broken is never observed).

The hug_var and associated structures in the driver, which aimed to improve memory usage in the driver by updating in place a HUG during upsweep, are no longer required as the HPT entries in the HUG are now themselves mutable by construction. This was previously explained in Note [ModuleNameSet, efficiency and space leaks], which is no longer relevant and was deleted.

Fixes #25511 (closed)

Co-authored-by: @mpickering

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    Metric Decrease:
        MultiComponentModulesRecomp
        MultiLayerModulesRecomp
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