Simplify Gradle build

It appears currently the project is built multiple times by CI with different JDK versions and different modularity settings. Maybe I am overlooking something, but I think the following should be done to simplify the build and to also increase its reproducibility:

• The source and target Java version (or better the --release version) should be fixed and independent of the JDK version a user (or the CI server) has installed; Gradle toolchains could be used to ensure this
• Instead there could be different test executions which alter the JUnit version and the runtime Java version. For that Gradle toolchains and possibly the incubating test suites feature could be used (maybe there are other solutions as well).
• The module-info.class should always be created, but it should be placed in a Multi-Release JAR folder. There are cases where build tools or plugins have issues with module-info.class being in the top-level directory of a Java 8 JAR file. The ModiTect plugin supports Multi-Release JARs by setting the jvmVersion value (should be set to 9 since that is the next higher version >= 9 after version 8 targeted by this project), see documentation. This would also allow running the Javadoc task with --release 9 then to have it generate a module overview, see also #566.

What do you think?

[aepfli]

i created a simply draft for now, for experiments, lets see if this is something we want to pursue

[nipafx]

On #614, @aepfli commented:

I would actually get first rid of all the modularity etc. [which happens during release of 2.0] and than continue from there, it is just a lot of bloat and will most likely need additional efforts, which is not really worth it, if we plan to upgrade to [Java] 11 anyways

So this won’t be in 2.0.