Replace "javac" attribute with "tools" attribute in java_toolchain.

This is preparation for removal of "javac" attribute.

The main usecase for "javac" attribute is to use it in $location function during JVM parameters expansion. And this works when javac is added to tools attribute.

There is a starlark API to retrieve javac from the java toolchain, but without a known usecase. That API is not used internally and it is not tested.

PiperOrigin-RevId: 342894757
diff --git a/tools/jdk/java_toolchain_default.bzl.java_tools b/tools/jdk/java_toolchain_default.bzl.java_tools
index c6e6e1c..356c9fe 100644
--- a/tools/jdk/java_toolchain_default.bzl.java_tools
+++ b/tools/jdk/java_toolchain_default.bzl.java_tools
@@ -62,7 +62,7 @@
 )
 
 JVM8_TOOLCHAIN_CONFIGURATION = dict(
-    javac = ["//:javac_jar"],
+    tools = ["//:javac_jar"],
     jvm_opts = ["-Xbootclasspath/p:$(location //:javac_jar)"],
 )
 
@@ -127,7 +127,6 @@
 
 _LABEL_LISTS = [
     "bootclasspath",
-    "extclasspath",
     "javac",
     "tools",
     "javabuilder",