Bug 578449 - Use same groupId in production pom than in consumer

Platform artifacts are published on Maven Central with
org.eclipse.platform
groupId.
To facilitate further work which could allow deploying to p2 and Maven
as
part of the same build, let's use the target/expected buildId directly
so
we don't have to generate a new one later.
artifactId is already 1-1 mapping.
Only versions would then be different between Maven (-SNAPSHOT) and p2
metadata
(.yyyyMMddhhmm)

Change-Id: I3a54021f23f165e11783d14feef7bf417b791f0f
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.resources/+/190248
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Mickael Istria <mistria@redhat.com>