Allow for using a `PatternFilterable` to determine source processed by `JavaExtension` and `KotlinExtension`

Sometimes you want your entire source code to be linted/formatted except for some code in a particular package.

For example, in my project I want everything reformatted except for things in files that match the pattern **/generated/**.

When setting up my java source sets, if I didn’t want these files to be compiled I could do something like this:

sourceSets {
    "main" {
        java {
            exclude("**/generated/**")
        }
    }
}

I propose that spotless should expose the same API for filtering out sources that I don’t want to be linted or reformatted.

I would expose an API that allowed the user to use a PatternFilterable to configure what is excluded from the task’s target FileCollection.

[JLLeitschuh]

It’s not time. It’s just that most of it lives on a git branch that has deviated so far from master that it’s going to be difficult to redo. I still have the branch checked out in a separate clone of the repo on my work machine.

I had to move a lot of code around to get it to work, ended up deleting some irrelevant projects for that the feature was making irrelevant, ect. That feature (actually more of a full project) got killed so the old code was still relevant.

I can port much of the same gradle logic, but I need to do the refactor and move code around dance all over again.

[nedtwigg]

Fixed in 3.18.0.