Support OutputPaths in OutputDirectory

Specifying any number of OutputPaths will ignore OutputFiles (consistent
with uploads). Where an OutputPath specifies an output directory, the
action must be able to create the directory itself.

src/main/java/build/buildfarm/worker/shard/CFCExecFileSystem.java

@@ -397,16 +397,25 @@ private Set<String> linkedDirectories(
     return ImmutableSet.of();
   }
 
+  private OutputDirectory createOutputDirectory(Command command) {
+    Iterable<String> files;
+    Iterable<String> dirs;
+    if (command.getOutputPathsCount() != 0) {
+      files = command.getOutputPathsList();
+      dirs = ImmutableList.of(); // output paths require the action to create their own directory
+    } else {
+      files = command.getOutputFilesList();
+      dirs = command.getOutputDirectoriesList();
+    }
+    return OutputDirectory.parse(files, dirs, command.getEnvironmentVariablesList());
+  }
+
   @Override
   public Path createExecDir(
       String operationName, Map<Digest, Directory> directoriesIndex, Action action, Command command)
       throws IOException, InterruptedException {
     Digest inputRootDigest = action.getInputRootDigest();
-    OutputDirectory outputDirectory =
-        OutputDirectory.parse(
-            command.getOutputFilesList(),
-            command.getOutputDirectoriesList(),
-            command.getEnvironmentVariablesList());
+    OutputDirectory outputDirectory = createOutputDirectory(command);
 
     Path execDir = root.resolve(operationName);
     if (Files.exists(execDir)) {