Prepare gemContents in a separate task

[dmikurube]

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)

[dmikurube]

build/gemContents is generated through the gem task now. Making it a separate task would be fine.