Correct task property annotations and some other miscellaneous fixes

[lptr]

Starting with version 6.0 Gradle will show deprecation warnings for tasks with incorrect annotations. The licenses plugin has a number of these, and in this PR I'm attempting to fix them. I also made some other changes:

• upgraded to Gradle 5.6.4 and enabled checks that will fail if task annotations are incorrectly defined
• turned task fields into Groovy properties, as annotations on fields are not supported in Gradle 6.0+
• removed the OssLicensesCleanupTask as it's not required: clean should already remove the whole build directory automatically
• no need to pass the outputDir to each task as Gradle takes care of creating parent directories for @OutputFile properties automatically
• made LicensesTask cacheable by default, and added the @PathSensitive annotation for its input file
• removed manually recorded inputs and outputs as they are already tracked via task property annotations

There is still the question of DependencyTask.configurations – this is essentially consuming the whole configuration container as input. This is probably not what is intended, but I left it as is. Someone with better understanding of the intention here should capture the inputs and perhaps make the task cacheable if possible in a separate PR.

There are some other things that could also be improved about the plugin in further PRs:

• use lazy task registration (instead of project.tasks.create() use project.tasks.register())
• use RegularFileProperty types for the task properties, so they can be connected to each other without having to manually specify dependsOn() relationships.

[filipwiech]

Any update on this?