Add support for predeclared dependencies.

[nedtwigg]

Starting in 6.0, dependencies are now resolved from project repositories rather than buildscript repositories. That fixed some things, but broke other things. In this PR, we add the ability to fix everything by predeclaring your formatters in the root project.

spotless {
  ...
  predeclareDeps()
}
spotlessPredeclare {
  java { eclipse() }
  kotlin { ktfmt('0.28') }
}

Alternatively, you can also use predeclareDepsFromBuildscript() to resolve the dependencies from the buildscript repositories rather than the project repositories.

If you use this feature, you will get an error if you use a formatter in a subproject which is not declared in the spotlessPredeclare block.

Fixes #1026, #1027, #1028, #1032, #1034, and #1042, by implementing #984

[JavierSegoviaCordoba]

What about repositories being set in dependencyResolutionManagement in settings file?

[nedtwigg]

That's a good question. I think they get hit by predeclareDepsFromBuildscript, which works like so.

spotless/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/GradleProvisioner.java

Lines 105 to 109 in 8ae601c

	static Provisioner forRootProjectBuildscript(Project project) {
	Project rootProject = project.getRootProject();
	ScriptHandler buildscript = rootProject.getBuildscript();
	return forConfigurationContainer(rootProject, buildscript.getConfigurations(), buildscript.getDependencies());
	}

If they don't get hit, I'm not sure how to hit them, but I'm open to a PR to add another mode or have them get included in the predeclareDepsFromBuildscript mode.

[nedtwigg]

Published in plugin-gradle 6.1.0.