Improves Modules Definition

We stuck with relative paths in 2.0. In 2.1, this should be:

<modules>
<module>
<groupId>...</groupId>
<artifactId>...</artifactId>
<version>...</version> (optional, obviously)
</module>
</modules>

This makes it the inverse of the <parent/> relationship, which makes sense. We would then have hinting for <relativePath/>, a search path for the modules (default being */pom.xml) etc.

Note that we can reuse the syntax, as the model version would be 4.1.0 and we can select the parser based on the model version (hopefully). To save confusion, perhaps we can find a more appropriately named tag, however.

This should all tie in to workspaces.