[MNG-8214] Improve model velocity template to support subclasses

[kwin]

This makes the constructor protected and takes the Builder as argument

Following this checklist to help us incorporate your
contribution quickly and easily:

• Make sure there is a JIRA issue filed
  for the change (usually before you start working on it). Trivial changes like typos do not
  require a JIRA issue. Your pull request should address just this issue, without
  pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [MNG-XXX] SUMMARY,
  where you replace MNG-XXX and SUMMARY with the appropriate JIRA issue.
• Also format the first line of the commit message like [MNG-XXX] SUMMARY.
  Best practice is to use the JIRA issue title in both the pull request title and in the first line of the commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the Core IT successfully.

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask on the developers list.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• In any other case, please file an Apache Individual Contributor License Agreement.

[gnodet]

I don't really see what's your use case for that ? We already have inheritance such as Build extends BuildBase and it works correctly. Is that just a cleanup ?

[kwin]

This is not about inheritance within MDO models but inheritance of classes generated from MDO, like for https://github.com/apache/maven-plugin-tools/blob/9332b09acc08d2e063f12a8158a57ca8ec56fadd/maven-plugin-tools-api/src/main/java/org/apache/maven/tools/plugin/ExtendedMojoDescriptor.java#L31. This is currently not possible due to the restricted visibility of the constructors and the lack of a constructor taking a builder as argument.

[gnodet]

This is not about inheritance within MDO models but inheritance of classes generated from MDO, like for https://github.com/apache/maven-plugin-tools/blob/9332b09acc08d2e063f12a8158a57ca8ec56fadd/maven-plugin-tools-api/src/main/java/org/apache/maven/tools/plugin/ExtendedMojoDescriptor.java#L31. This is currently not possible due to the restricted visibility of the constructors and the lack of a constructor taking a builder as argument.

Fair point. However the use case of ExtendedMojoDescriptor is definitely a bad one. Those were needed because the models did not really evolved, but we allowed small additions without properly versioning them, and then rely on such hacks.

[kwin]

However the use case of ExtendedMojoDescriptor is definitely a bad one.

I only partially agree, extending the MDO for this case is not useful as the additional attribute really only is useful in the plugin-tools context and therefore this class is only used internally. Although one could probably do things differently, the effort when not extending the original Maven classes is considerably higher (as one somehow needs to establish the connection between the raw Maven object and the extended plugin tools one).

[gnodet]

The $additionalArguments needs to be removed

[gnodet]

The locations field should be immutable.