[JaxRS] Code generator does not honor implFolder on windows platform

Description

When using jaxrs-cxf-cdi and other JaxRS generators, the implFolder config is not honored by hte generator on windows.

The issue is due to file path manipulations in AbstractJavaJAXRSServerCodegen:apiFilename that uses '/' as a separator, while DefaultCodeGen:apiFilename uses File.separator since e7f4fb3c453e08582c9b955d64321fce8cb1e539 introduced for #7808

Swagger-codegen version

Tested on 2.3.1 and 2.4.0-SNAPSHOT, and this is a regression w.r.t. 2.2.2

Related issues/PRs

The issue that seemingly introduced the regression: https://github.com/swagger-api/swagger-codegen/issues/6443

Suggest a fix/enhancement

Use File.separator in AbstractJavaJAXRSServerCodegen:apiFilename

 @Override
  public String apiFilename(String templateName, String tag) {
      String result = super.apiFilename(templateName, tag);

      if ( templateName.endsWith("Impl.mustache") ) {
          int ix = result.lastIndexOf(File.separator);
          result = result.substring(0, ix) + "/impl" + result.substring(ix, result.length() - 5) + "ServiceImpl.java";
          result = result.replace(apiFileFolder(), implFileFolder(implFolder));
      } else if ( templateName.endsWith("Factory.mustache") ) {
          int ix = result.lastIndexOf(File.separator);
          result = result.substring(0, ix) + "/factories" + result.substring(ix, result.length() - 5) + "ServiceFactory.java";
          result = result.replace(apiFileFolder(), implFileFolder(implFolder));
      } else if ( templateName.endsWith("Service.mustache") ) {
          int ix = result.lastIndexOf('.');
          result = result.substring(0, ix) + "Service.java";
      }
      return result;
  }

Start in AbstractJavaJAXRSServerCodegen.apiFilename and compare its path handling with DefaultCodeGen.apiFilename, particularly for Impl.mustache and Factory.mustache templates. Verify the change with a Windows JaxRS generator configuration using implFolder, and confirm generated implementation files are placed in that folder as they were in version 2.2.2.