Flatten Documenter module hierarchy

[mortenpi]

I think the time has come. @fredrikekre? I think the only modules we should keep around are the actual modules under utilities/ (like DOM).

While it should be straightforward and non-functional, and our tests should catch most errors, it may still lead to some accidental breakage. Also, sometimes users might be referring to internal functions with fully qualified module paths, which will no-longer work. Hence, good to do this in 0.28.