Fix javac plugin compilation under JDK 17+

JavaToolchainPlugin (now removed) used to mask two JDK-17+ issues in the
javacPlugin project by pinning its compile JDK to 11 and stripping flags
from the doc task. With the plugin gone, both issues surface:

1. JDK 14 added Plugin.autoStart(), so JDK 17/21 javac eagerly enumerates
   ServiceLoader<Plugin> providers from the compile classpath. During
   incremental compilation our META-INF/services/com.sun.source.util.Plugin
   descriptor lives on the classpath but SemanticdbPlugin.class isn't
   built yet, so ServiceLoader throws 'Provider ... not found'.

   Fix: move the descriptor from src/main/resources/ to
   src/main/assembly-resources/ and inject that directory only into the
   assembled fat jar via 'assembly / fullClasspath'. The descriptor is
   absent from the compile classpath but still bundled in the published
   jar.

2. javadoc rejects the '-g' flag that was previously added via
   'javacOptions += "-g"'. The old plugin worked around this with
   '(doc / javacOptions) --= List("-g")'.

   Fix: scope '-g' to 'Compile / javacOptions' so it never reaches the
   doc task.

Author: jupblb

build.sbt

@@ -124,7 +124,16 @@ lazy val javacPlugin = project
     fatjarPackageSettings,
     javaOnlySettings,
     moduleName := "semanticdb-javac",
-    javacOptions += "-g",
+    Compile / javacOptions += "-g",
+    // The META-INF/services/com.sun.source.util.Plugin descriptor lives in
+    // src/main/assembly-resources/ instead of src/main/resources/ so that
+    // it ends up in the assembled fat jar but is NOT on the compile
+    // classpath. Otherwise JDK 14+ javac eagerly loads Plugin providers via
+    // ServiceLoader during compilation and trips on the not-yet-compiled
+    // SemanticdbPlugin class.
+    assembly / fullClasspath += Attributed.blank(
+      baseDirectory.value / "src" / "main" / "assembly-resources"
+    ),
     (assembly / assemblyShadeRules) :=
       Seq(
         ShadeRule