skipIfSpecIsUnchanged doesn't work if the spec is not in the project

Created by: osvetlik

https://github.com/OpenAPITools/openapi-generator/blob/c224cf484b020a7f5997d883cf331715df3fb52a/modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/CodeGenMojo.java#L451

When checking the original hash, you don't read the spec file when it is in a dependency rather then in the project itself. Later on, when calculating the new one, you take care to read it wherever it might be. This way you have the hash stored, but you never use it and the API is generated every time. This is very annoying for me, because my Eclipse keeps on generating sources and building the new ones and so on.

Could you fix this or do you want me to do it?