Add project example for subpackages

[vabenil]

The documentation for dub subpackages as the rest of the dub docs are lacking in examples.
Adding a repository with simple dub examples would massively help beginners getting into D and thereafter dub.

To be more specific my reason for opening this issue is because of the lack of mention of the file structure of a subpackage. In the dub subpackage documentation in this example:

Does not include that the source directory for component1 or component2 should be ./component/source/component/ whereas a beginner might assume that the correct path should ./component/source/.

Had there been an example dub project where I could have seen the correct file structure I would not have wasted hours with this issue.

[WebFreak001]

for current new docs deployment see https://deploy-preview-54--dub-registry.netlify.app/

[rtvd]

It is definitely still not clear how subpackages are supposed to work.

For example, a natural thing to be would seem the following:

1. A root package acting as a group. So as it has no code whatsoever, I would expect its targetType to be "none".
2. Various sub-packages containing libraries and executables which depend on those libraries.

However, I found several problems:

1. It is not clear where should the dependencies be defined (and while not every subpackage may need every single dependency, when they do use one, they should better use the same version). Should the dependencies be defined in the parent, outermost package? Or in each subpackage (causing a tautology and possible divergence)? Or perhaps in a special subpackage that would contain no code and would serve only as a way to transitively provide dependencies to other subpackages?
2. if I have a root package with targetType=none and a subpackage then dub build fails as unlike mvn from Java or cargo from Rust it builds the root package only unless told otherwise. dub :mysubpackage works. Is it OK? Not quite. This does not work well with D Language plugin for IntelliJ because it needs to figure out the location of dependencies and dub describe shows nothing. It also does not show that there is a "mysubpackage" to deal with. Now if you run dub describe :mysubpackage - you can see the dependencies. But tooling has no obvious way to know about that subpackage in the first place.
3. So I thought maybe I'll add dependency ":mysubpackage" version="*" to the root dub.sdl. With that, the build works. However, dub describe fails with core.exception.AssertError@source/dub/compilers/dmd.d(283): No target name set..

It seems dub expects the root package to build at least something in order for it to depend on something, which does not always make sense. And if it does not depend on the subpackages then dub describe is not helpful.