Minor cleanup: invert parent pom and bom location in file-system

In order to have a more concise BOM, relocate it to a sub-module and
move the parent POM to the parent directory. Logically, the BOM is still
the parent of the parent POM, but it now no longer needs to include
project management related information such as license, scm,
repositories, etc.

(manually reviewed by Gaetan)