Add support for project isolation.

[NikolayMetchev]

The plugin doesn't appear to be compatible with project isolation (https://docs.gradle.org/current/userguide/isolated_projects.html) :
The problem is with this bit of code here:

ktlint-gradle/plugin/src/main/kotlin/org/jlleitschuh/gradle/ktlint/TaskCreation.kt

Line 45 in 818098d

parentProject.plugins.withId("org.jlleitschuh.gradle.ktlint") {

[wakingrufus]

Thanks for bringing this up.
I think we will need to remove this bit, and if you want your root build script linted, you will need to apply the plugin directly there. We will need to investigate if this could cause other issues

[JLLeitschuh]

• Fix gradle deprecation warning #480
• ccc2ee1

@pvegh looks like that pig came back looking for it's lipstick 😆

#480 (comment)