Add --post-compile-merge-dir option in zinc (#7917)

Prerequisite for #7869, so resources like
```
META-INF/services/
META-INF/services/javax.annotation.processing.Processor
```
in the z.jar can be added by zinc wrapper instead of post processing by Pants.

src/scala/org/pantsbuild/zinc/compiler/Main.scala

@@ -10,6 +10,7 @@ import scala.collection.JavaConverters._
 
 import sbt.internal.inc.IncrementalCompilerImpl
 import sbt.internal.util.{ ConsoleLogger, ConsoleOut }
+import sbt.io.IO
 import sbt.util.Level
 import xsbti.CompileFailed
 
@@ -160,6 +161,11 @@ object Main {
       // Run the compile.
       val result = new IncrementalCompilerImpl().compile(inputs, log)
 
+      // post compile merge dir
+      if (settings.postCompileMergeDir.isDefined) {
+        IO.copyDirectory(new File(settings.postCompileMergeDir.get.toURI), new File(settings.classesDirectory.toURI))
+      }
+
       // Store the output if the result changed.
       if (result.hasModified) {
         targetAnalysisStore.set(

src/scala/org/pantsbuild/zinc/compiler/Settings.scala

@@ -28,6 +28,7 @@ case class Settings(
   _sources: Seq[File]               = Seq.empty,
   classpath: Seq[File]              = Seq.empty,
   _classesDirectory: Option[File]   = None,
+  _postCompileMergeDir: Option[File] = None,
   outputJar: Option[File]           = None,
   scala: ScalaLocation              = ScalaLocation(),
   scalacOptions: Seq[String]        = Seq.empty,
@@ -47,6 +48,8 @@ case class Settings(
   lazy val classesDirectory: File =
     normalise(_classesDirectory.getOrElse(defaultClassesDirectory()))
 
+  lazy val postCompileMergeDir: Option[File] = _postCompileMergeDir.map(normalise)
+
   lazy val incOptions: IncOptions = {
     _incOptions.copy(
       apiDumpDirectory = _incOptions.apiDumpDirectory map normalise,
@@ -282,6 +285,10 @@ object Settings {
       .action((x, c) => c.copy(classpath = x))
       .text("Specify the classpath")
 
+    opt[File]("post-compile-merge-dir")
+      .action((x, c) => c.copy(_postCompileMergeDir = Some(x)))
+      .text("Directory to merge with compile outputs after compilation")
+
     opt[File]("class-destination")
       .abbr("d")
       .action((x, c) => c.copy(_classesDirectory = Some(x)))