Deprecate relativePath

The concept of relativePath is alien to the overall Maven design of having project directory that only depends on entities in the repositories. With relative-paths the build might yield to different results depending on were a project folder is located in the local filesystem.

The parent POM resolution was changed in Maven 3. Because of this explicit relativePaths need to be specified more often for reactor builds to be built in the correct order. The reason for this (according to Maven 3.x compatibility note) is to improve consistency: "In Maven 2, building the child project in isolation could fail while the reactor build would succeed to resolve the parent.". However this behaviour is inconsistent with the resolution of the other dependencies, in fact the above is true for any Maven version when a dependency that is part of the reactor is not available in a suitable versions in the repository: in this case the build of the individual project fails while the build of the whole reactor succeeds.

Because of this relativePath should be marked as deprecated and the parent should be treated like a dependency when computing the build order of reactor projects.