chore(spin-common): unify source-path detection behind SourcePathKind

[deer]

Every plugin that needed to know where source lived -- Java25CompilerPlugin, Java8CompilerPlugin, Java25JUnitPlugin, Java8JUnitPlugin, CheckstylePlugin, MavenPlugin -- carried its own near-duplicate DetectSourceFiles/DetectSourcePaths/DetectGeneratedSourcePaths/DetectExternalGeneratedSourcePaths/DetectAllSourcePaths task interfaces and abstract base classes, six of which did nothing but repeat the same directory-walking logic under a different name.

This collapses all of that into a single SourcePathKind enum (MAIN, TEST, GENERATED, EXTERNAL_GENERATED) in spin-common, plus one DetectSourcePaths task interface whose detect() now returns Map<SourcePathKind, PathSet>. Each plugin's AbstractDetectSourcePaths subclass only declares which kinds it's responsible for; consumers pull out just the kinds they need via the new DetectSourcePaths.filesOf/pathsOf static helpers. The interface is tagged @Category("source-paths") and @Merge, so a @From(DetectSourcePaths.class) Map<SourcePathKind, PathSet> parameter automatically receives the combined result of every implementor in the project rather than requiring callers to know which concrete plugin classes exist.

This also fixes a real gap in CheckstylePlugin: previously, honoring a project's <includeTestSourceDirectory> setting required a private helper that duplicated AbstractDetectSourceFiles's walk logic and hardcoded the src/test/java convention rather than going through the same TEST-kind detection JUnitPlugin uses. It now asks for SourcePathKind.MAIN/TEST from the merged map like everything else, and AbstractResourcePlugin.DetectResourcePaths was decoupled from DetectSourcePaths entirely, since resource directories were never actually "source paths" and would no longer type-check against the new Map-returning interface.