Fix recursive Ctags output

Previously using `dscanner --ctags -R <dir>` would output tags without
any filenames, making it pretty useless for navigating in a project.
This was due to all the syntax objects being merged into a single generic
Module, then outputting that module without a filename specified.

This fix does the following:
 * Moves the CTag header output from the Module class to the proper spot in
    main.d
 * Renames `Module.writeCtagsTo` to `getCtags` and now returns string[]
    with the prepared tag lines.
 * The CTag printing functionality in main.d now gathers all tag lines
    from each module, sorts, then outputs.

Note that a more optimal implementation would be fairly straightforward.
Instead of simply returning a string[], `Module.toCtags` could take a
sorted container and insert in sorted order.  main.d could pass this
container to each module in turn, then output the results.