Fix javac plugin compilation under JDK 17+

JavaToolchainPlugin (now removed) used to mask three JDK-17+ issues in
the javacPlugin project by pinning its compile JDK to 11, stripping flags
from the doc task, and forcing --release 11 on all Java code. With the
plugin gone, all three surface:

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

   Fix: pass an explicit empty -processorpath to javac so it doesn't scan
   our own output directory for Plugin providers. The descriptor stays in
   src/main/resources/ so that internal sbt consumers (e.g. the
   'minimized' test project that depends on javacPlugin via dependsOn)
   and the Bazel build can still find it.

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.

3. sbt-assembly's shader ships an older ASM that cannot read class major
   version 61 (JDK 17). When sbt runs under JDK 17/21 the fat jar ends up
   with SemanticdbVisitor (and other classes that hit protobuf types)
   silently un-shaded, so at runtime javac fails with
   'NoClassDefFoundError: com/google/protobuf/ProtocolMessageEnum'.

   Fix: add 'Compile / javacOptions ++= Seq("--release", "11")' to
   javaOnlySettings so Java code always emits class version 55, which the
   shader handles, regardless of which JDK runs sbt.

Author: jupblb

build.sbt

@@ -124,7 +124,20 @@ lazy val javacPlugin = project
     fatjarPackageSettings,
     javaOnlySettings,
     moduleName := "semanticdb-javac",
-    javacOptions += "-g",
+    Compile / javacOptions += "-g",
+    // JDK 14+ javac eagerly enumerates Plugin providers from the processor
+    // path (or, if absent, the compile classpath). During incremental
+    // compilation our own META-INF/services/com.sun.source.util.Plugin
+    // resource is on the classpath but SemanticdbPlugin.class isn't built
+    // yet, which trips ServiceLoader. Force an explicit empty processor
+    // path so javac doesn't scan our own output directory. The resource
+    // must stay in src/main/resources/ so that internal sbt dependents
+    // (e.g. the `minimized` test project) can find the plugin descriptor.
+    Compile / javacOptions ++= {
+      val empty = target.value / "empty-processorpath"
+      IO.createDirectory(empty)
+      Seq("-processorpath", empty.getAbsolutePath)
+    },
     (assembly / assemblyShadeRules) :=
       Seq(
         ShadeRule
@@ -635,7 +648,12 @@ lazy val javaOnlySettings = List[Def.Setting[_]](
   incOptions ~= { old =>
     old.withEnabled(false).withApiDebug(true)
   },
-  crossPaths := false
+  crossPaths := false,
+  // Java 11 is our minimum supported runtime and sbt-assembly's shader uses
+  // an ASM version that can't read newer class files (e.g. class major 61
+  // emitted by JDK 17). Pinning the release target keeps the bytecode at
+  // version 55 regardless of which JDK runs sbt, so shading still works.
+  Compile / javacOptions ++= Seq("--release", "11")
 )
 
 val testSettings = List(