Drop elabGPkgDescription from the elaborated plan

[PaleRoses]

Fixes #12463.

drops elabGPkgDescription; the in-library configure reads the .cabal file instead, as Setup does. unpacking already writes revisions there.

no-op cabal build all, one package with N public sublibraries; master 538d465, ghc 9.14.1, medians of three:

N	improved-plan	no-op
10	0.79 → 0.59 MB	0.08 → 0.07 s
40	7.5 → 4.4 MB	0.33 → 0.25 s
80	27.3 → 14.8 MB	1.04 → 0.65 s

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary. (none needed)
• Manual QA notes have been included.
• Tests have been added. (no behaviour change; existing tests cover the configure)

QA Notes

build a package with internal libraries, and one from Hackage with a revision; both configure as before.

[sheaf]

This MR attempts to regain sharing after-the-fact during serialisation. I think that's papering over the issue in an incomplete and non-robust way.

As far as I understand, the regression was due to the addition of elabGPkgDescription to ElaboratedConfiguredPackage. So the fix should be to remove that field and find another way to thread that information through which avoids the duplication.

A similar win in this area is to stop elabPkgDescription being duplicated in every single unit of a package, which could be similarly achieved by removing it from the unit-specific data and shared, but that change is not strictly needed to address the regression.

[PaleRoses]

done: dropped elabGPkgDescription, the in-library configure reads the .cabal file like Setup does. n=80 repro: improved-plan 27.3 → 14.8 MB. happy to do the same for elabPkgDescription in a follow-up.

[sheaf]

Yes, much better approach, thanks. We will need to think through what it means for error messages for users, e.g. ensure that it doesn't cause duplicate error messages.