Fix broken build on Windows OS if the schema contains references to other schema files. (#21300)

Author: akuhtz

modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java

@@ -563,12 +563,18 @@ public void execute() throws MojoExecutionException {
         }
 
         if (StringUtils.isNotBlank(inputSpecRootDirectory)) {
+            // make sure the path can be processed correct under Windows OS
+            inputSpecRootDirectory = inputSpecRootDirectory.replaceAll("\\\\", "/");
+
             inputSpec = new MergedSpecBuilder(inputSpecRootDirectory, mergedFileName,
                     mergedFileInfoName, mergedFileInfoDescription, mergedFileInfoVersion)
                     .buildMergedSpec();
             LOGGER.info("Merge input spec would be used - {}", inputSpec);
         }
 
+        // make sure the path can be processed correct under Windows OS
+        inputSpec = inputSpec.replaceAll("\\\\", "/");
+
         File inputSpecFile = new File(inputSpec);
 
         if (output == null) {