Display plugin updates reports extensions as plugins without a defined version

[andrzejj0]

See #1182

.mvn/extensions.xml:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.1.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.1.0 https://maven.apache.org/xsd/core-extensions-1.1.0.xsd">
  <extension>
    <groupId>me.qoomon</groupId>
    <artifactId>maven-git-versioning-extension</artifactId>
    <version>9.8.1</version>
  </extension>
</extensions>

.mvn/maven-git-versioning-extension.xml:

<configuration xmlns="https://github.com/qoomon/maven-git-versioning-extension"
               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xsi:schemaLocation="https://github.com/qoomon/maven-git-versioning-extension https://qoomon.github.io/maven-git-versioning-extension/configuration-9.4.0.xsd">
  <refs>
    <ref type="branch">
      <pattern>.+</pattern>
      <version>${ref}-SNAPSHOT</version>
      <properties>
        <foo>${ref}</foo>
      </properties>
    </ref>
    <ref type="tag">
      <pattern><![CDATA[v(?<version>.*)]]></pattern>
      <version>${ref.version}</version>
    </ref>
  </refs>
  <!-- optional fallback configuration in case of no matching ref configuration-->
  <rev>
    <version>${commit}</version>
  </rev>
</configuration>

mvn versions:display-plugin-updates

Undesired output:

[WARNING] The following plugins do not have their version specified:
[WARNING]   me.qoomon:maven-git-versioning-extension ................. 9.8.1

Also, extensions are treated as plugins and the mojo lists new available versions:

[INFO] Require Maven 3.0 to use the following plugin updates:
[INFO]   me.qoomon:maven-git-versioning-extension ........... 9.8.1 -> 5.0.2
[INFO] 
[INFO] Require Maven 3.6.3 to use the following plugin updates:
[INFO]   me.qoomon:maven-git-versioning-extension ........... 9.8.1 -> 9.6.3
[INFO] 
[INFO] Require Maven 3.6.4 to use the following plugin updates:
[INFO]   me.qoomon:maven-git-versioning-extension .......... 9.8.1 -> 9.10.0

[andrzejj0]

Reopening: that particular extension happens to also be a plugin, so this is a very special case. Regular extensions do not get reported.

Status of this:

• Maven 4.0.0: the extension/plugin does not get reported by versions-maven-plugin 2.17.1 as a plugin
• Maven 3.x.x: it does, but that's only because this particular extension is also a plugin.

The question is: why this isn't reported with Maven 4?
How to treat this particular case. Should it be shown? But it's defined as a plugin... Especially in case of a core extension, it's not even present in pom.xml files.

My idea is: just to make the behaviour consistent across Maven 3->4, I will still be removing the plugins that have an extension with the same GAV. But other than that, that's pretty much it.

Thoughts? @slawekjaranowski @slachiewicz