Fix package path computation in ClasspathScanner

Prior to this commit no trailing `/` character was
appended to the computed package path.

Now, except for the default package `""`, a `/` is
appended to package path. This leads to corrected
and documented behavior even if two modules start
with the same name elements.

Fixes #2500

documentation/src/docs/asciidoc/release-notes/release-notes-5.8.0-M1.adoc

@@ -15,7 +15,8 @@ on GitHub.
 
 ==== Bug Fixes
 
-* ❓
+* Method `scanForClassesInPackage(String)` in `ClasspathScanner` now returns a valid list
+  of class names when the package name is equal to the name of a module on the module path.
 
 ==== Deprecations and Breaking Changes
 

junit-platform-commons/src/main/java/org/junit/platform/commons/util/ClasspathScanner.java

@@ -211,7 +211,11 @@ private ClassLoader getClassLoader() {
 	}
 
 	private static String packagePath(String packageName) {
-		return packageName.replace(PACKAGE_SEPARATOR_CHAR, CLASSPATH_RESOURCE_PATH_SEPARATOR);
+		if (packageName.isEmpty()) {
+			return "";
+		}
+		String path = packageName.replace(PACKAGE_SEPARATOR_CHAR, CLASSPATH_RESOURCE_PATH_SEPARATOR);
+		return path + CLASSPATH_RESOURCE_PATH_SEPARATOR;
 	}
 
 	private List<URI> getRootUrisForPackage(String basePackageName) {

platform-tests/src/test/java/org/junit/platform/commons/util/ClasspathScannerTests.java

@@ -33,6 +33,7 @@
 import java.util.function.Predicate;
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
+import java.util.spi.ToolProvider;
 import java.util.stream.Collectors;
 
 import org.junit.jupiter.api.Test;
@@ -180,22 +181,45 @@ void scanForClassesInPackage() {
 
 	@Test
 	// #2500
-	void scanForClassesInPackageWithinModuleSharingNames() throws Exception {
-		var jarfile = getClass().getResource("/com.greetings@1-ea.jar");
+	void scanForClassesInPackageWithinModulesSharingNamePrefix(@TempDir Path temp) throws Exception {
+		var moduleSourcePath = Path.of(getClass().getResource("/modules-2500/").toURI()).toString();
+		run("javac", "--module", "foo,foo.bar", "--module-source-path", moduleSourcePath, "-d", temp.toString());
 
-		var module = "com.greetings";
+		checkModules2500(ModuleFinder.of(temp)); // exploded modules
+
+		var foo = temp.resolve("foo.jar");
+		var bar = temp.resolve("foo.bar.jar");
+		run("jar", "--create", "--file", foo.toString(), "-C", temp.resolve("foo").toString(), ".");
+		run("jar", "--create", "--file", bar.toString(), "-C", temp.resolve("foo.bar").toString(), ".");
+
+		checkModules2500(ModuleFinder.of(foo, bar)); // jarred modules
+
+		System.gc(); // required on Windows in order to release JAR file handles
+	}
+
+	private static int run(String tool, String... args) {
+		return ToolProvider.findFirst(tool).orElseThrow().run(System.out, System.err, args);
+	}
+
+	private void checkModules2500(ModuleFinder finder) {
+		var root = "foo.bar";
 		var before = ModuleFinder.of();
-		var finder = ModuleFinder.of(Path.of(jarfile.toURI()));
 		var boot = ModuleLayer.boot();
-		var configuration = boot.configuration().resolveAndBind(before, finder, Set.of(module));
+		var configuration = boot.configuration().resolve(before, finder, Set.of(root));
 		var parent = ClassLoader.getPlatformClassLoader();
 		var layer = ModuleLayer.defineModulesWithOneLoader(configuration, List.of(boot), parent).layer();
 
-		var classpathScanner = new ClasspathScanner(() -> layer.findLoader(module), ReflectionUtils::tryToLoadClass);
-
-		var classes = classpathScanner.scanForClassesInPackage("com.greetings", allClasses);
-		var classNames = classes.stream().map(Class::getName).collect(Collectors.toList());
-		assertThat(classNames).hasSize(1).contains("com.greetings.Main");
+		var classpathScanner = new ClasspathScanner(() -> layer.findLoader(root), ReflectionUtils::tryToLoadClass);
+		{
+			var classes = classpathScanner.scanForClassesInPackage("foo", allClasses);
+			var classNames = classes.stream().map(Class::getName).collect(Collectors.toList());
+			assertThat(classNames).hasSize(2).contains("foo.Foo", "foo.bar.FooBar");
+		}
+		{
+			var classes = classpathScanner.scanForClassesInPackage("foo.bar", allClasses);
+			var classNames = classes.stream().map(Class::getName).collect(Collectors.toList());
+			assertThat(classNames).hasSize(1).contains("foo.bar.FooBar");
+		}
 	}
 
 	@Test

platform-tests/src/test/resources/modules-2500/foo.bar/FooBar.java

@@ -0,0 +1,3 @@
+package foo.bar;
+
+public class FooBar {}

platform-tests/src/test/resources/modules-2500/foo.bar/module-info.java

@@ -0,0 +1,3 @@
+open module foo.bar {
+  requires foo;
+}

platform-tests/src/test/resources/modules-2500/foo/Foo.java

@@ -0,0 +1,3 @@
+package foo;
+
+public class Foo {}

platform-tests/src/test/resources/modules-2500/foo/module-info.java

@@ -0,0 +1,3 @@
+module foo {
+  exports foo;
+}

platform-tooling-support-tests/src/test/java/platform/tooling/support/tests/JavacModulesTests.java

@@ -100,6 +100,7 @@ private static List<String> compileModules(Path temp, Writer out, Writer err, Fu
 		try (var walk = Files.walk(base)) {
 			var projects = walk.filter(path -> path.endsWith("module-info.java")) //
 					.map(base::relativize) //
+					.filter(path -> !path.startsWith("platform-tests")) //
 					.filter(path -> !path.startsWith("platform-tooling-support-tests")) //
 					.map(base::resolve) //
 					.map(Project::new) //