Unable to find resource 'xxxxxxxxxxxx:xxxxx:maven-plugin:x.x.x' in repository central

Although one may think to correct this by adding a <repository> specification in the POM file, a closer look at the error messages, including "A required plugin was not found: Plugin could not be found - check that the goal name is correct: Unable to download the artifact from any repository" may give another clue.  For plugins, a <pluginRepository> specification is required.