Refactor org.openapitools.codegen package

Hi, @wing328 @spacether

Issue Description

Package org.openapitools.codegen is a promiscuous package, and groups together miscellaneous functionalities that might be useful to different subsystems. The package structure violates the “high cohesion and low coupling” design rules. I found that Class CliOption and SpecValidationException is not used by classes in their package. The project has grown larger, leading to becoming increasingly hard to maintain. For example, if one modifies package org.openapitools.codegen (i.e., package rename), resulting in multiple classes of package org.openapitools.codegen.languages to be changed.

Location: The source file can be found at path File modules/openapi-generator/src/main/java/org/openapitools/codegen/CliOption.java
modules/openapi-generator/src/main/java/org/openapitools/codegen/SpecValidationException.java

Refactoring suggestions

I suggest to move class CliOption into package org.openapitools.codegen.languages, SpecValidationException into package org.openapitools.codegen.config.

Start with modules/openapi-generator/src/main/java/org/openapitools/codegen/CliOption.java and SpecValidationException.java, then inspect references to both classes across the generator. Done means CliOption is in org.openapitools.codegen.languages, SpecValidationException is in org.openapitools.codegen.config, and affected references compile with the new package locations.