[MNG-7646] Do not parse all projects in the reactor when building a subtree

[gnodet]

• [MNG-7642] Restore compatibility with flatten-maven-plugin
• Add some comment about deprecation and remove the unused field
• [MNG-7652] switch to Modello Velocity ([MNG-7652] switch to Modello Velocity #944)
• [MNG-7642] Explicit the fact that the class is now a no-op class
• Disable parsing the whole reactor

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[gnodet]

Wrong PR

[jira-importer]

Resolve #8807