Move server-specific classes into server-specific packages

[wilkinsona]

[thecooldrop]

@wilkinsona This sounds like a relatively simple task, which is good to get to know the code base. I would like to continue contributing to Spring Boot. I would like to work on this task if possible

[wilkinsona]

Thanks for the offer, @thecooldrop, but I'm already working on this. Generally speaking, anything labelled with theme: structure Issues related to the structure of the project and its code will be better-handled by the core team.