Separate two types of internal check

* Starting with uri:classloader:/jruby/kernel are filenames that
  come from JRuby's internal Ruby sources. They should gain the
  internal prefix.
* Starting with internal prefix indicates an internal source that
  may be appropriate to exclude under certain conditions.

We check both of these at trace-construction time because the
relevant rubyspec shows the behavior to also filter out user-
provided filenames with the same internal prefix, which is used
to test the masking and removal of these lines. In order to
handle both true internal filenames and user-provided internal-
prefixed filenames, both must be checked for omission here.

I don't agree with the side effect of being able to mask any trace
line, but I think that train has sailed.

core/src/main/java/org/jruby/runtime/backtrace/BacktraceData.java

@@ -2,7 +2,6 @@
 
 import com.headius.backport9.stack.StackWalker;
 import org.jruby.Ruby;
-import org.jruby.runtime.builtin.IRubyObject;
 import org.jruby.util.JavaNameMangler;
 
 import java.io.Serializable;
@@ -93,7 +92,7 @@ private RubyStackTraceElement[] constructBacktrace(Map<String, Map<String, Strin
             // Only rewrite non-Java files when not in "raw" mode
             if (!(rawTrace || filename == null || filename.endsWith(".java"))) {
                 // skip internal sources if requested
-                if (excludeInternal && TraceType.isInternal(filename)) continue;
+                if (excludeInternal && TraceType.isExcludedInternal(filename)) continue;
 
                 List<String> mangledTuple = JavaNameMangler.decodeMethodTuple(methodName);
                 if (mangledTuple != null) {
@@ -173,7 +172,7 @@ private RubyStackTraceElement[] constructBacktrace(Map<String, Map<String, Strin
 
                 // skip internal sources if requested
                 filename = rubyFrame.filename;
-                if (excludeInternal && TraceType.isInternal(filename)) continue;
+                if (excludeInternal && TraceType.isExcludedInternal(filename)) continue;
 
                 // mask internal file paths
                 filename = TraceType.maskInternalFiles(filename);

core/src/main/java/org/jruby/runtime/backtrace/TraceType.java

@@ -603,16 +603,23 @@ public static IRubyObject generateMRIBacktrace(Ruby runtime, RubyStackTraceEleme
     }
 
     public static String maskInternalFiles(String filename) {
-        if (isInternal(filename)) {
+        if (isInternalFile(filename)) {
             return "<internal:" + filename + ">";
         }
 
         return filename;
     }
 
-    public static boolean isInternal(String filename) {
-        return filename.startsWith("uri:classloader:/jruby/kernel/") ||
-                filename.startsWith("<internal:");
+    public static boolean isInternalFile(String filename) {
+        return filename.startsWith("uri:classloader:/jruby/kernel/");
+    }
+
+    public static boolean hasInternalMarker(String filename) {
+        return filename.startsWith("<internal:");
+    }
+
+    public static boolean isExcludedInternal(String filename) {
+        return TraceType.isInternalFile(filename) || TraceType.hasInternalMarker(filename);
     }
 
     @Deprecated