Mark SourceDirectorySet.outputDir as Incubating

[oehme]

This API is new in 4.0, but wasn't marked @Incubating. This is even worse since this API is definitely wrong, so we want to change it. We should mark it incubating for 4.0.1, so no one starts depending on it.

[big-guy]

I don't think we should deprecate an existing API without providing another stable API (if we think we should support the use case) to move to. That was the thinking for not making SourceDirectorySet.outputDir incubating (right or wrong).

[oehme]

Alternatively we should fix the design, but that might be a tall order for a hotfix release. As discussed with @wolfs, this should go on SourceSetOutput, just like the methods for resource directories.

[jjohannes]

@wolfs @oehme can one of you make the change, reflecting what you two discussed, to the hotfix branch: https://github.com/gradle/gradle/commits/release_4.0.1 (or prepare a PR based on the branch). Thanks!

[oehme]

I've marked it as incubating and changed the deprecation message to point to a non-incubating API (telling users to change the corresponding compile tasks's output dir).

[jjohannes]

Thanks @oehme!

Any remaining concerns @big-guy @wolfs?

Otherwise I assume this is good to go.

[wolfs]

Good to go from my side.