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

```- [ ] summary of problem

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:

com.diffplug.spotless

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:

Some Awesome Maven Plugin
somePrefix
some-maven-plugin

- [ ] 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.

com.diffplug.spotless
spotless-maven-plugin
1.21.0

- [ ] 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

[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)
```

首先使用插件配置重现 `mvn -s settings.xml spotless:check`，并将其与可正常工作的完全限定 goal 进行比较。检查 `com.diffplug.spotless:spotless-maven-plugin` 的发布方式，以及是否生成了组级别的 Maven 元数据。当 `spotless` 前缀能够解析且 check goal 成功运行时，即表示完成。

由索引模型根据 Issue 内容生成。