Fix Windows path separators in legacy directory error message

Normalize backslashes to forward slashes in the physical presence
error message to ensure consistent output across platforms.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ascheman

impl/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java

@@ -1008,7 +1008,7 @@ private void failIfLegacyDirectoryPresent(
                             "Legacy directory '%s' exists but cannot be used in project %s "
                                     + "because sources are configured via <sources>. "
                                     + "Remove or rename the directory.",
-                            defaultPath, projectId);
+                            defaultPath.toString().replace('\\', '/'), projectId);
                     logger.error(message);
                     result.getProblemCollector()
                             .reportProblem(new org.apache.maven.impl.model.DefaultModelProblem(