Support using Gradle projects as the dependency inputs

I'd finished Ktlint custom rule sets support in #1896, but it supports Maven coordinates only due to the limits of

https://github.com/diffplug/spotless/blob/0189f717ee13acdb00745bffa86c6ed4ede53ef2/lib/src/main/java/com/diffplug/spotless/JarState.java#L56-L59

We can extend this for Gradle local projects to be used as the inputs, not sure if this way works for Maven projects as well.

Read lib/src/main/java/com/diffplug/spotless/JarState.java at lines 56-59 and review the Ktlint custom rule set work in #1896. Trace how Maven coordinates currently become dependency inputs, then determine how a Gradle local project can provide the same input. Done means Gradle projects are accepted as inputs while the existing Maven-coordinate path remains usable.

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