Gradle plugin: don't register generated sources via generatedKotlin on KSP projects

[oldergod]

Problem

Since 7.0.0-alpha01 (#3573), on Kotlin 2.3+ the Gradle plugin registers its generated Kotlin sources via KotlinSourceSet.generatedKotlin instead of kotlin.srcDir(). KSP (KSP1 and KSP2 alike) doesn't know about generatedKotlin, which has two effects on projects applying KSP:

1. Wire-generated sources are invisible to symbol processors — they are no longer part of the source roots KSP collects.
2. KSP tasks lose their task dependency on the Wire task. KSP wires its dependencies from sourceSet.kotlin; the kotlin.srcDir(provider) registration was what carried the edge to generateProtos. Builds which also reference Wire's output directory from another source set — a common real-world pattern, e.g. sourceSets.main.java.srcDir(layout.buildDirectory.dir("generated/source/wire")) — now fail Gradle's execution-time dependency validation:

A problem was found with the configuration of task ':kspKotlin' (type 'KspAATask').
  - Gradle detected a problem with the following location: '…/build/generated/source/wire'.
    Reason: Task ':kspKotlin' uses this output of task ':generateMainProtos' without declaring an
    explicit or implicit dependency.

Found rolling Wire 7.0.0-alpha06 out to Cash's server monorepo, where this failed exactly as above on Kotlin/KSP 2.3.10 with Gradle 9.4.

Fix

When the com.google.devtools.ksp plugin is applied, fall back to the kotlin.srcDir() registration — the Wire 6 behavior every KSP user had. Projects without KSP keep the generatedKotlin registration and its IDE benefits. This mirrors the Android-side decision from #3611, where sources are registered on variant.sources.java specifically so KSP can see them.

The long-term resolution is for KSP to support generatedKotlin (collect its directories and carry their task dependencies); this fallback can be removed once it does.

Testing

New kotlin-project-kotlin-protos-kgp23-ksp fixture: KGP 2.3.10 + KSP2 2.3.10 (ksp.useKSP2=true) with a Moshi codegen processor, plus the plain-path source-set registration pattern from the wild. Against the unfixed plugin it fails with exactly the validation error above; with the fix, the fixture and the two existing generatedKotlin/fallback tests all pass. The fix was also verified against the failing Cash monorepo module by republishing the plugin locally.