Add an option to automatically exclude other project directories from target(...)

[vlsi]

It is quite easy to configure allprojects { autostyle {..., however, it results in duplicate processing of the same files.

allprojects { // <-- !!!
    autostyle {
        format("markdown") {
            target("**/*.md")
            trimTrailingWhitespace()
        }
    }
}

It turns out target("**/*.md") affects sub-projects as well.

In case for Apache JMeter we have a bunch of modules, and it would be great if `"**/*.md" did not descend to subprojects.
For instance, the root project should validate its own files, then subprojects should validate their own files, and so on.

I might see someone might want a "flat rule" when the rule is declared at the root project and verifies all the files, however, it is not that convenient because in that case I can no longer invoke spotlessCheck to check all the files relevant for the current submodule.

[vlsi]

The solution is to operate on the files relevant to one project by default.

The escape hatch is excludeSubprojects.set(false).