[cli][gradle] Adds package name as a global option (#2557)

* [cli] Support packageName as global option

CLI supports other package-related options (--api-package,
--model-package, --invoker-package). This aligns those options with
--package-name to avoid confusion about how to configure these options
where those are supported.

This intentionally does not apply a packageName getter/setter to
DefaultCodegen to reduce the footprint of this change.

* [gradle] Add packagName as global option

This makes packageName available as a global option, beside apiPackage,
modelPackage, and invokerPackage to reduce potential confusion about how
to configure the four of these options by generators which support them.

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenConstants.java

@@ -137,6 +137,8 @@ public class CodegenConstants {
 
     public static final String PROJECT_NAME = "projectName";
     public static final String PACKAGE_NAME = "packageName";
+    public static final String PACKAGE_NAME_DESC = "package for generated classes (where supported)";
+
     public static final String PACKAGE_VERSION = "packageVersion";
 
     public static final String PACKAGE_TITLE = "packageTitle";