Allow configuring files that will be packaged under META-INF

It is useful to configure files that will be packaged under META-INF (not META-INF/vault ). For instance, in the Sling project we require that legal artifacts ( META-INF/LICENSE and META-INF/NOTICE ) are present in the main artifacts.

There are possible workarounds involving the maven-resources-plugin, but a simple solution from this plugin would be better.

Initially discussed at https://lists.apache.org/thread.html/raacee511c1e6dbf79ffeb2021736d27b76ce6a8a5f25a528df4492b7%40%3Coak-dev.jackrabbit.apache.org%3E