UseJava: Move helper scripts to subdirectory

Also, exclude them from the help module index.

Modules/UseJava/Symlinks.cmake

@@ -0,0 +1,20 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+# Helper script for UseJava.cmake
+
+if (UNIX AND _JAVA_TARGET_OUTPUT_LINK)
+    if (_JAVA_TARGET_OUTPUT_NAME)
+        find_program(LN_EXECUTABLE
+            NAMES
+                ln
+        )
+
+        execute_process(
+            COMMAND ${LN_EXECUTABLE} -sf "${_JAVA_TARGET_OUTPUT_NAME}" "${_JAVA_TARGET_OUTPUT_LINK}"
+            WORKING_DIRECTORY ${_JAVA_TARGET_DIR}
+        )
+    else ()
+        message(SEND_ERROR "FATAL: Can't find _JAVA_TARGET_OUTPUT_NAME")
+    endif ()
+endif ()