pom: Change where default if groupid is set

Since groupid is a new key for -pom, it is safe to use a different
default for where when the groupid key is set. In this case the default
is META-INF/maven/${groupid}/${artifactid}/pom.xml instead of pom.xml.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>