Supporting dedicated configuration for groovy source.
The configuration allows access to formatter steps which have been introduced for Java, but which are also useable for Groovy code.
Integrates Groovy-Eclipse based Groovy formatter step implementation.
Allows coexistent configuration of Java and Groovy formatter steps.

…Set<String> has complicated serialization. Reused FileSignature.toSortedSet() for a more stable serialization.

Also, using FileSignature.toSortedSet() without first calling asNonNulList() would be an error.  So, might as well have toSortedSet() call that method itself as its first argument.

…ion its own invisible <a> tag for stable links.

…and SpotlessExtension. This means that Spotless can now use greclipse to format its gradle files.

The configuration allows access to formatter steps which have been introduced for Java, but which are also useable for Groovy code.
Integrates Groovy-Eclipse based Groovy formatter step implementation.
Allows coexistent configuration of Java and Groovy formatter steps.

… greclipse formatter)

…expected formatter output.

…ct to test. Omit implicit testing of end-line characters for licenseHeader step.

EXCLUDE_JAVA_DEFAULT was being used as an alias for `false`.  If we changed its value to true, it didn't just change the default behavior, it would also break the implementation logic.

…enCentral as well.

…an reference it directly.

The DSL should be based on the name of the project.  Many formatters have "format" in the name -> google-java-format, scalafmt, etc.  But others don't, e.g. ktlint.