Prepare gemContents in a separate task

Some Embulk plugins have been quick-tested with the `-l` option for `lib/`. As the `lib/` directory is removed when this Gradle plugin is applied, we need an alternative.

Reference:
* https://github.com/civitaspo/embulk-filter-expand_json/tree/v0.3.0#run-example
* https://twitter.com/Civitaspo/status/1226525105154646016 (ja_JP)

Start by inspecting the Gradle plugin's task handling and how the `lib/` directory is removed when the plugin is applied. Use the linked Embulk plugin quick-test examples as the expected behavior; done means `gemContents` is prepared in a separate task and provides an alternative to using `-l` with `lib/`.

Written by the indexing model from the issue text.