Kapt3: Check if the 'tools.jar' is present in the plugin classpath (KT-17456)

Kapt3 won't work without the 'tools.jar' file provided, but we shouldn't throw an exception in this case.
This commit adds the warning message (and disables Kapt if it can't be used).

plugins/kapt3/src/org/jetbrains/kotlin/kapt3/Kapt3Plugin.kt

@@ -159,6 +159,10 @@ class Kapt3CommandLineProcessor : CommandLineProcessor {
 }
 
 class Kapt3ComponentRegistrar : ComponentRegistrar {
+    private companion object {
+        private const val JAVAC_CONTEXT_CLASS = "com.sun.tools.javac.util.Context"
+    }
+
     fun decodeOptions(options: String): Map<String, String> {
         val map = LinkedHashMap<String, String>()
 
@@ -184,6 +188,13 @@ class Kapt3ComponentRegistrar : ComponentRegistrar {
                                ?: PrintingMessageCollector(System.err, MessageRenderer.PLAIN_FULL_PATHS, isVerbose)
         val logger = KaptLogger(isVerbose, messageCollector)
 
+        try {
+            Class.forName(JAVAC_CONTEXT_CLASS)
+        } catch (e: ClassNotFoundException) {
+            logger.warn("'$JAVAC_CONTEXT_CLASS' class can't be found ('tools.jar' is absent in the plugin classpath). Kapt won't work.")
+            return
+        }
+
         val sourcesOutputDir = configuration.get(Kapt3ConfigurationKeys.SOURCE_OUTPUT_DIR)?.let(::File)
         val classFilesOutputDir = configuration.get(Kapt3ConfigurationKeys.CLASS_OUTPUT_DIR)?.let(::File)
         val stubsOutputDir = configuration.get(Kapt3ConfigurationKeys.STUBS_OUTPUT_DIR)?.let(::File)