[java] Support module import declarations (JEP 476)

- new JModuleSymbol
- UnnecessaryImportRule also supports module imports
- module-info.class can be loaded by ClasspathClassLoader
- Support annotations on modules
- Verify that exported packages are loaded

pmd-test/src/main/java/net/sourceforge/pmd/test/RuleTst.java

@@ -12,6 +12,11 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.StringWriter;
 import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.security.CodeSource;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -220,6 +225,27 @@ public abstract class RuleTst {
         return runTestFromString(test.getCode(), rule, test.getLanguageVersion());
     }
 
+    private static final Path PATH_TO_JRT_FS_JAR;
+
+    static {
+        // find jrt-fs.jar to be added to auxclasspath
+        // Similar logic like jdk.internal.jrtfs.SystemImage
+        CodeSource codeSource = Object.class.getProtectionDomain().getCodeSource();
+        if (codeSource == null) {
+            PATH_TO_JRT_FS_JAR = Paths.get(System.getProperty("java.home"), "lib", "jrt-fs.jar");
+        } else {
+            URL location = codeSource.getLocation();
+            if (!"file".equalsIgnoreCase(location.getProtocol())) {
+                throw new IllegalStateException("Object.class loaded in unexpected way from " + location);
+            }
+            try {
+                PATH_TO_JRT_FS_JAR = Paths.get(location.toURI());
+            } catch (URISyntaxException e) {
+                throw new IllegalStateException(e);
+            }
+        }
+    }
+
     /**
      * Run the rule on the given code and put the violations in the report.
      */
@@ -229,6 +255,8 @@ public abstract class RuleTst {
         configuration.setDefaultLanguageVersion(languageVersion);
         configuration.setThreads(0); // don't use separate threads
         configuration.prependAuxClasspath(".");
+        configuration.prependAuxClasspath(PATH_TO_JRT_FS_JAR.toString());
+
 
         try (PmdAnalysis pmd = PmdAnalysis.create(configuration)) {
             pmd.files().addFile(TextFile.forCharSeq(code, FileId.fromPathLikeString("file"), languageVersion));