Migrate the generator drivers' per-package manifest logic to consume hydra.build.manifest

[joshsh]

Following #560 (routing driver migration — the proven consume-then-delete pattern), promote the next pure-core unit under #559: per-package manifest field logic.

Each host driver still hand-codes the manifest-writing logic that assembles a package's fields (mainModules / testModules / mainDslModules / mainEncodingModules / package) into its manifest.json:

• Java: Generation.writePackageManifests (heads/java/src/main/java/hydra/Generation.java:968)
• Python: write_package_manifests (heads/python/src/main/python/hydra/generation.py:834)
• Haskell: the equivalent in the Haskell driver.

The generated hydra.build.manifest module (packages/hydra-build/src/main/haskell/Hydra/Sources/Build/Manifest.hs, already landed in the hydra-build package) encodes this field-assembly/serialization logic translingually — the drivers should consume it rather than duplicate it.

Scope

Flip the three drivers to consume hydra.build.manifest for the manifest field-assembly + serialization decision logic, deleting the hand-coded duplication — same two-part pattern as #560 (Haskell shim delegating to generated -> Java -> Python), with the same hard constraints:

1. dist/json byte-identical. writePackageManifests's emitted manifest.json (alphabetized fields, sorted namespace arrays) must not move a byte. Drift elimination, not behavior change.
2. Both Java compile paths validated — the local (json-driver/build.gradle) and the published (:hydra-java:compileHeadsExtrasJava) compiles must pass. This is the coverage gap that caused Migrate the three generator drivers to consume hydra.build.routing; delete the legacy static prefix tables #560's RED-MAIN (CI runs the published cold-clone compile, which local /test java doesn't exercise). If the driver newly references a hydra.build.manifest symbol, wire it into both paths.
3. Driver-level fail-loud tests for any Either-to-exception wrappers, per host (mirroring Migrate the three generator drivers to consume hydra.build.routing; delete the legacy static prefix tables #560's Step 5).

Sequencing

Pure-core, touches hydra.build.* + the drivers only — orthogonal to hydra.lib.*, so drivable now around #417's breaking wave (still out). Natural continuation of #560 (writePackageManifests was one of #560's rewired call sites).

Relates to: #416, #559, #560. Lesson applied from #560: validate the published compile path, not just local /test.

[joshsh]

Parent: #559 (host-agnostic build milestone). Second pure-core driver-migration child after #560 (routing).

[joshsh]

Resolved: the generator drivers' per-package manifest logic now consumes the generated `hydra.build.manifestWriter` module (landed via `727ecb7dc8` — the restore of the migration + cold-seeder shim after two earlier cold-clone reverts).

Validated green: the Cold-clone Haskell sync job — the step that reverted this twice — passed on CI run 30162152744, with `727ecb7dc8` an ancestor of the green tip.

Note: a follow-up hardening item (extend `bin/check-oil-and-water.py` to flag `ColdSeedMain`-transitive imports of unpublished `Hydra.Build.*` modules, preventing this revert class) is tracked separately under staging's direction — it is not part of #607's migration scope, which is complete.