Move the definition into the main platform repository
Update findDuplicates to get spring-boot-dependencies pom from a repo

Previously, findDuplicates took two arguments: the path to 
spring-boot-dependencies’ pom and the path to the platform’s pom. This
commit updates it to obtain the former from
repo.spring.io/libs-snapshot, having determined the required version by
looking in the platform definition file, and to automatically use
platform-bom/pom.xml for the latter
Move buildPlatform script into the main platform repository
Remove management of dependencies that are now handled by Boot