Maven can't locate this plugin due to "org.apache.maven.plugin.prefix.NoPluginFoundForPrefixException: No plugin found for prefix 'spotless'"

Start by reproducing mvn -s settings.xml spotless:check with the plugin configuration and compare it with the working fully qualified goal. Inspect how com.diffplug.spotless:spotless-maven-plugin is published and whether the group-level Maven metadata is generated. Done means the spotless prefix resolves and the check goal runs successfully.

When try to run "mvn -s settings.xml spotless:check", it can't locate this plugin due to "org.apache.maven.plugin.prefix.NoPluginFoundForPrefixException: No plugin found for prefix 'spotless'"

I even entered the plugin group info in settings.xml:
<pluginGroups>
  <pluginGroup>com.diffplug.spotless</pluginGroup>
</pluginGroups>

But maven refuses to find this plugin via prefix. Full naming works though (com.diffplug.spotless:spotless-maven-plugin:check).

For the prefix to work my understanding is that the plugin should provide prefix info in its maven-metadata.xml at the group level. But this plugin does not come with such meta data file. It only has maven-metadata.xml at the artifact level for each artifact version. That is, I could not find maven-metadata.xml which content like this:

<metadata>
  <plugins>
    <plugin>
      <name>Some Awesome Maven Plugin</name>
      <prefix>somePrefix</prefix>
      <artifactId>some-maven-plugin</artifactId>
    </plugin>
  </plugins>
</metadata>

- [ ] gradle or maven version

Maven 3.5.0

- [ ] spotless version

1.27.0 but I tried many other versions

- [ ] operating system and version

Ubuntu Xenial

- [ ] copy-paste your full Spotless configuration block(s), and a link to a public git repo that reproduces the problem if possible

It's very simple.

<plugin>
    <groupId>com.diffplug.spotless</groupId>
    <artifactId>spotless-maven-plugin</artifactId>
    <version>1.21.0</version>
    <configuration>
        <java>
            <removeUnusedImports/>
        </java>
    </configuration>
</plugin>

- [ ] copy-paste the full content of any console errors 

[DEBUG] Resolving plugin prefix spotless from [com.diffplug.spotless, org.apache.maven.plugins, org.codehaus.mojo]
[DEBUG] Using mirror repo (http://scctools.dt.vmware.com:8081/artifactory/repo) for plexus-snapshots (https://oss.sonatype.org/content/repositories/plexus-snapshots).
[DEBUG] Using mirror repo (http://scctools.dt.vmware.com:8081/artifactory/repo) for apache.snapshots (http://repository.apache.org/snapshots).
[DEBUG] Using mirror repo (http://scctools.dt.vmware.com:8081/artifactory/repo) for codehaus.org (http://snapshots.repository.codehaus.org).
[DEBUG] Could not find metadata com.diffplug.spotless/maven-metadata.xml in local (/home/jenkins/.m2/repository)
[DEBUG] Could not find metadata org.apache.maven.plugins/maven-metadata.xml in local (/home/jenkins/.m2/repository)
[DEBUG] Could not find metadata org.codehaus.mojo/maven-metadata.xml in local (/home/jenkins/.m2/repository)
[DEBUG] Using transporter WagonTransporter with priority -1.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
[DEBUG] Using transporter WagonTransporter with priority -1.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
[DEBUG] Using transporter WagonTransporter with priority -1.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
[DEBUG] Using connector BasicRepositoryConnector with priority 0.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
[DEBUG] Using connector BasicRepositoryConnector with priority 0.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
[DEBUG] Using connector BasicRepositoryConnector with priority 0.0 for http://scctools.dt.vmware.com:8081/artifactory/repo
Downloading: http://scctools.dt.vmware.com:8081/artifactory/repo/org/apache/maven/plugins/maven-metadata.xml
Downloading: http://scctools.dt.vmware.com:8081/artifactory/repo/org/codehaus/mojo/maven-metadata.xml
Downloading: http://scctools.dt.vmware.com:8081/artifactory/repo/com/diffplug/spotless/maven-metadata.xml
[DEBUG] Writing tracking file /home/jenkins/.m2/repository/com/diffplug/spotless/resolver-status.properties

<snipped....>

[DEBUG] Could not find metadata com.diffplug.spotless/maven-metadata.xml in repo (http://scctools.dt.vmware.com:8081/artifactory/repo)
[DEBUG] Could not find metadata com.diffplug.spotless/maven-metadata.xml in local (/home/jenkins/.m2/repository)
[DEBUG] Skipped remote request for com.diffplug.spotless/maven-metadata.xml, already updated during this session.
[DEBUG] Failure to find com.diffplug.spotless/maven-metadata.xml in http://scctools.dt.vmware.com:8081/artifactory/repo was cached in the local repository, resolution will not be reattempted until the update interval of repo has elapsed or updates are forced
[DEBUG] Could not find metadata org.apache.maven.plugins/maven-metadata.xml in local (/home/jenkins/.m2/repository)
[DEBUG] Could not find metadata org.codehaus.mojo/maven-metadata.xml in local (/home/jenkins/.m2/repository)