qdoc should generate HTML and DITA XML output in subdirectories according to the module structure

qdoc generates all the documentation in a single output directory. In Qt 5.0, the Qt functionality is modularized, but the documentation can't be modularized simply by separating documentation files by module. because documentation files in one module can contain links to documentation files in other modules. This means that to get a set of documentation with no broken links, you must have all the Qt modules present when you run qdoc. But we this may not be satisfactory for certain packages of Qt that contain only a subset of the modules. To begin to solve this problem, we should change qdoc to generate its output files in subdirectories according to the module structure. Then packages of subsets of modules can include only the documentation subdirectories appropriate for the specific packages, and broken links to documentation pages in modules that are not included in the package can be handled more gracefully than with a brutal PAGE NOT FOUND error.