Dogfood native test fixtures
This commit replaces our custom test fixtures with the native "Java test fixtures".
The `TestFixturesPlugin` of our build has been simplified to leverage the native
test fixtures capabilities. Some noticeable changes:
- the `testFixtures` extension has been replaced with regular dependency declaration
- dependencies on test fixtures are now declared using the `testFixtures` keyword
- test fixtures properly declare an API and an implementation, meaning that the
implementation dependencies of test fixtures no longer leak into the compile
classpath of consumers (in particular tests)
Change all subprojects to use 'implementation' dependencies
- 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
- 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'
- 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.