[MNG-8650] Provide a tool to validate a POM

**[Guillaume Nodet](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=gnt)** opened **[MNG-8650](https://issues.apache.org/jira/browse/MNG-8650?redirect=false)** and commented

I think we should provide a simple command line that can be used to validate a POM and report warnings and errors in a simple way.
We could then ask Sonatype to make sure that POMs uploaded to Maven Central do not contain any error and raise the warnings back to maintainers.

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No further details from [MNG-8650](https://issues.apache.org/jira/browse/MNG-8650?redirect=false)

The issue names no files, tests, or existing entry points. Start by locating Maven's command-line and POM validation components; the work is complete when a simple command reports POM warnings and errors in a form suitable for maintainers and Maven Central checks.

Written by the indexing model from the issue text.