Modularization

[patrickkusebauch]

Closes #924

The idea is to split the application into independent parts. Clearly define the stable public API. Along the way, dogfood our own features like private collectors and sample extensions.

I have build this PR to be reviewed commit by commit, otherwise I think it will be too large to review.

The best place to start on the first commit is the deptrac.yaml file. One layer per directory, no fancy collector magic.

[dbrumann]

As discussed, we should probably discuss the namespace changes in an Issue. Otherwise we run the risk of moving code around every so often. I'd much rather have an issue we can refer to for why the directory/namespace structure is the way it is, in case someone proposes changes.

[dbrumann]

Sorry for taking so long to review this. Looks good to me. 👍

Just as a heads up, I might revert your change in the DependencyEmitters switching from getAlias to the key again at some point. I want to get rid of checking for certain keys outside the class itself, like we do with EmitterTypes. I am afraid that this will cause issues with extensions at some point or force them to overwrite our keys instead of providing their own. By not using them we will (hopefully) make sure, that our code is not restricted or behave weirdly when a new, unknown emitter type is introduced by an extension.