Deprecation warning using recent Gradle 4.4.1

[twwwt]

Using a recent Gradle version 4.4.1, I get the following warning on the console:

Using TaskInputs.file() with something that doesn't resolve to a File object has been deprecated and is scheduled to be removed in Gradle 5.0. Use TaskInputs.files() instead.

Seems that a minor refactoring of the plugin is due.