Decide when is appropriate for a module to have an includes directory

[jenlampton]

Follow-up to #486

Most modules in core do not have an includes directory, but now that we have directories to neatly organize many other types of files in a module, some have expressed interest in adding an includes directory too.

I personally dislike this idea, because I want to know from looking inside only the module's root directory exactly what that module is up to. However, if we look at views as an example, if all it's PHP files were in the root, the sheer number of files would be overwhelming, so some order is required.

We should decide on a good rule-of-thumb for when modules should further organize their files into an includes directory (perhaps a certain number of files?) and make sure core abides by this rule of thumb.