MergeMavenParents parses local accessible parent pom.xml or fetches it from upstream, merges into root project, then interpolate the properties.

Maven looks for the parent POM first in 'relativePath', then the local repository '../pom.xml', and lastly in the remote repo.

ProjectKey returns a project key with empty groupId/version filled by corresponding fields in parent.

MaxParent sets a limit on the number of parents to avoid indefinite loop.