Previously we had a dependency to the tests package. As the
nullreader package is in the pkg directory it should be publicly
available. However, having the tests dependency could lead to a
transitive dependency to an internal package, if the tests package
would import one. Thus, we removed it.
We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout
