faster -PspotlessIdeHook with configure-on-demand

Right now, the IDE hook works like this

```
spotlessApply -PspotlessIdeHook=${ABSOLUTE_PATH_TO_FILE}
```

But for large multiproject projects we could make it significantly faster with (note the leading colon)

```
:spotlessApply -PspotlessIdeHook=${ABSOLUTE_PATH_TO_FILE} --configure-on-demand
```

but only if we modified this code https://github.com/diffplug/spotless/blob/48723eb15b3e6b0ada0841bd78e4dd271af9507f/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessExtensionImpl.java#L54-L56

by adding something like:

```java
if (project == rootProject) {
String ideHookPath = (String) project.property(IdeHook.PROPERTY);
project.gradle.settingsEvaluated { settings ->
// the code below works only for root with children, needs to also handle children with children
ProjectDescriptor owningProject = null;
ProjectDescriptor rootDescriptor = settings.rootProject
for (ProjectDescriptor child : rootDescriptor.children) {
if (ideHookPath.startsWith(child.projectDir.absolutePath)) {
owningProject = child;
}
}
if (owningProject != null) {
project.evaluationDependsOn(owningProject.path)
}
}
}
```

In this way we can avoid evaluating all the `spotlessApply` tasks and instead only evaluate the root project and, if necessary, the project which contains the file in question.