fix: include plugin classpath in action cache keys - #9632

All committers have signed the CLA.

What was broken

The action cache key for a cached task is sha256(codeContentHash, hash(inputs), extraHash, cacheVersion), where codeContentHash is the hash of the task body's macro tree (Cont.scala) and extraHash is always 0. Nothing in the key reflects the metabuild classpath. A plugin like sbt-scalac-opts-plugin defines scalacOptions ++= scalacOptsFor(scalaVersion.value, scalacOptsAll) — the tree is identical across plugin versions, only the data behind it changes. So after a version bump:

1. scalacOptions (and compile) replayed stale cached values;
2. after cleanFull, a recompile with the stale options failed and the CachedCompileFailure was recorded under a key that ignores the plugin classpath;
3. every subsequent compile replayed that failure for a configuration that compiles fine; only cleanFull recovered.

How it's fixed

• On each project load, two digests are computed from already-available content hashes and passed via BuildWideCacheConfiguration:
  • metaBuildDigest: every build unit's pluginData.classpath (plugin/dependency jars + compiled project/*.scala products). Mixed into every cached task key, so any plugin or build-helper change invalidates dependent cached values.
  • buildDefinitionDigest: the above plus the Eval-generated .sbt classes. Used only to key cached failures: a failure is stored under sha256(inputDigest, buildDefinitionDigest) and can only replay while the whole build definition is byte-identical. Failure entries written by older sbt under the loose key are treated as a miss and overwritten.
• The .sbt dsl classes are deliberately excluded from the global key so a plain build.sbt edit doesn't invalidate the entire cache (guarded by the existing cache/version-bump-compile scripted test).

Verified with the repro from https://github.com/stasimus/sbt2-stale-cache-repro: all README steps produce the expected exit codes with this patch. New scripted test cache/plugin-scalac-options-toggle covers the toggle scenario; utilCache/test, scripted cache/*, and MiMa all pass.