Check the version of the parent POM, too

When doing a multi-module build, Maven checks that the coordinates of the parent POM matches what you'd get by reading the POM via relativePath but it doesn't check the version.

This can cause subtle bugs if the coordinates are correct but one forgets to update the version of all modules after the parent version changes.

Please print a warning if the version in the <parent> element doesn't match the version of the POM read via relativePath.