Switch to multi-modules Maven project

[desruisseaux]

The project is currently a single Maven project, without modules. In support for #40 we may need to switch to a multi-modules Maven project. A project layout proposal is on the wiki. This issue is about the minimal change for allowing implementation of #40 in separated modules. The layout proposal for this issue is:

proj-platform
└─ proj

The two other modules proposed in the layout wiki page (proj-bin and proj-wasm) would be the subject of a separated issue.