Ensure nohttp checkstyle is only applied at top level

[wilkinsona]

The sourceDirectories configuration of our Checkstyle setup mean that modules will check the entire directory tree beneath their location, including any sub-modules. In the 2.x builds this means that checking in spring-boot-build covers the entire source tree. We need to find a way to check more than just .java files, but avoid checking nested modules.

[philwebb]

I think we can flip this on its head and have the main build check all subfolders and just not inherited it in the children. I think I have it working locally but we'll need some CI updates.