Adapt bazel-jdt-java-toolchain to latest ecj changes

See https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2577, some changes were merged to JDT core to support bazel-jdt-java-toolchain.

bazel-jdt-java-toolchain doesn't seem to support those changes yet though, when using both latest ecj and bazel-jdt-java-toolchain, we get this error:

  Exception in thread "main" java.lang.NoClassDefFoundError: org/eclipse/jdt/internal/compiler/batch/FileSystem$ClasspathAnswer
          at com.google.devtools.build.buildjar.javac.BlazeEcjMain$BlazeEclipseBatchCompiler.getLibraryAccess(BlazeEcjMain.java:173)
          at org.eclipse.jdt.core.compiler.batch/org.eclipse.jdt.internal.compiler.batch.Main.performCompilation(Main.java:4675)
          at org.eclipse.jdt.core.compiler.batch/org.eclipse.jdt.internal.compiler.batch.Main.compile(Main.java:1679)
          at com.google.devtools.build.buildjar.javac.BlazeEcjMain.compile(BlazeEcjMain.java:331)
          at com.google.devtools.build.buildjar.ReducedClasspathJavaLibraryBuilder.compileSources(ReducedClasspathJavaLibraryBuilder.java:58)
          at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.compileJavaLibrary(SimpleJavaLibraryBuilder.java:120)
          at com.google.devtools.build.buildjar.SimpleJavaLibraryBuilder.run(SimpleJavaLibraryBuilder.java:128)
          at com.google.devtools.build.buildjar.BazelEcjJavaBuilder.build(BazelEcjJavaBuilder.java:110)
          at com.google.devtools.build.buildjar.BazelEcjJavaBuilder.parseAndBuild(BazelEcjJavaBuilder.java:82)
          at com.google.devtools.build.buildjar.BazelEcjJavaBuilder.main(BazelEcjJavaBuilder.java:70)
  Caused by: java.lang.ClassNotFoundException: org.eclipse.jdt.internal.compiler.batch.FileSystem$ClasspathAnswer
          at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
          at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
          at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
          ... 10 more

Start by reading Eclipse JDT core pull request 2577 and reproducing the reported failure with the latest ECJ and bazel-jdt-java-toolchain. Trace BlazeEcjMain.getLibraryAccess and the missing FileSystem$ClasspathAnswer class in the stack trace. Done means the toolchain works with the ECJ changes without this NoClassDefFoundError.