Reflect the change of the repository name

This assumes that the pull request will be merged just before or after
renaming the repository from 'maven-nar-plugin' to 'nar-maven-plugin'
as per the convention that 'maven-*-plugin' names are reserved for
the special plugins that Maven Central controls.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Rename the groupId

In the same spirit as renaming the plugin from 'maven-nar-plugin'
to 'nar-maven-plugin' as per the convention that only Maven Central
plugins should follow the 'maven-*-plugin' (some plugins are more
equal than others), we also follow the convention to use a groupId
that we 'control'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>