Make the bootstrap plugin more consistent with the code base

[gsmet]

• rename it to quarkus-bootstrap-maven-plugin
• reformat pom.xml (in a separate commit)
• move the descriptors to META-INF/ instead of having a quarkus/ directory at the root.

[aloubyansky]

Looks good. One thing I remember we wanted to consider was to use io.quarkus as a groupId. We should probably discuss that. But that could be done in a separate PR. Thanks!

[gsmet]

If it's an entirely independent project, I think it would make sense to have a separate groupId but, yes, better discuss it on the list. Just underline in the email that it is supposed to be an independent project.