Support sbt 2.0.0 (drop sbt 1.x cross-build)

[xerial]

Summary

Resumes the sbt 2 migration now that sbt 2.0.0 is officially released, and completes it by targeting sbt 2.0.0 / Scala 3.8.4 exclusively. Cross-building back to sbt 1.x is dropped — the source-level differences (Scala 3 syntax, sbt 2 cached-task output types, Def.uncached escape hatch, fileConverter plumbing) make a single cross-built source tree impractical.

Plugin changes

• Bump to sbt 2.0.0 and Scala 3.8.4 (matches sbt 2.0.0's bundled compiler — older Scala 3 versions can't read sbt 2.0.0's TASTy 28.8).
• Wrap pack tasks in Def.uncached(...) — opts out of sbt 2's typed cache, which doesn't accept File/Path outputs or custom return types without JsonFormat.
• Drop Def.derive wrappers on file-producing tasks. sbt 2's automatic dynamicFileOutputs broadcasts the derivations into delegated scopes where the underlying task isn't defined, causing build-load failures.
• Hardcode Runtime config for packLibJars / packModuleEntries (was reading configuration.value, which is now unresolved at project scope after dropping Def.derive).
• packTargetDir defaults to baseDirectory.value / "target" instead of target.value — sbt 2's target.value resolves to a nested target/out/jvm/u/{project}/ path, not the stable location users expect.
• Plumb sbt's fileConverter through PluginCompat so HashedVirtualFileRef paths resolve via the build's MappedFileConverter (expands ${OUT} / ${BASE} placeholders) instead of the placeholder-leaving PlainVirtualFileConverter.

Layout & tooling

• Consolidate scala-2/ and scala-3/ PluginCompat into a single src/main/scala/ tree.
• Update .scalafmt.conf dialect to scala3.
• Drop sbt 1.x CI job; simplify command to ./sbt test scripted.

Scripted test fixes

• Replace force () infix invocation with .force() (Scala 3 rejects the Scala 2 nullary-method infix form).
• nested-project: distinct project names + crossPaths := false to avoid sbt 2's "Overlapping output directories" check.
• copy-dependencies: anchor packCopyDependenciesTarget to baseDirectory.value / "target" for the same reason as packTargetDir.
• Remove sbt 1.x pins on jvm-version-opts and exclude-test-config — they now run under sbt 2.

Test plan

• ./sbt scalafmtCheckAll — clean
• ./sbt test — 5 unit tests pass
• ./sbt scripted — all 17 scripted tests pass under sbt 2.0.0
• CI green on Java 21 and Java 24

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request migrates the sbt-pack plugin to Scala 3 and sbt 2.0.0. Key changes include replacing Def.derive with Def.uncached, integrating sbt 2.x's FileConverter for virtual file compatibility, and updating target directory definitions to avoid nested output directories. Feedback suggests updating packCopyDependenciesTarget to use packTargetDir.value / "lib" instead of target.value / "lib" to maintain consistency with the updated packTargetDir and ensure dependencies are copied to the correct location.

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[gemini-code-assist]

Since packTargetDir was updated to default to baseDirectory.value / "target" to avoid sbt 2.0's nested output directory, packCopyDependenciesTarget should also be updated to use packTargetDir.value / "lib" instead of target.value / "lib". This ensures that copied dependencies are placed in the stable, user-expected target/lib directory by default, maintaining consistency with packTargetDir and sbt 1.x behavior.

    packCopyDependenciesTarget           := packTargetDir.value / "lib",