testing-junit-platform.gradle.kts

Change all subprojects to use 'implementation' dependencies
This includes:
- All projects now explicitly declare all dependencies to other
subprojects. This makes issues more visible, guards for accidental
addition of new dependencies, and leaks much less transitive
dependencies on the compile classpathes.
- All usages of 'runtime' to declare dependencies have been replaced
with 'runtimeOnly'
- All projects are now `java-library` (and declare this explicitly)
- Most remaining Groovy scripts are translated to Kotlin
- The old 'compile' and 'runtime' configurations are not
configured/created anymore for the 'testFixture' and 'integTest'
source sets.
- Some obsolete dependencies (see previous commits) are removed
- 'api' is used scarcely on purpose as the current project structure is
not well designed for this. The projects contain code for several
concerns of the build tool and thus putting complete projects on the
API of others exposes too much. They should be split up along
functional concerns first.