[MNG-7761] Something funky is happening with m-site-p in Maven

**[Tamas Cservenak](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=cstamas)** opened **[MNG-7761](https://issues.apache.org/jira/browse/MNG-7761?redirect=false)** and commented

Not an issue per se (hence Task), but something funky is happening with site plugin, as noted in comments here: https://github.com/apache/maven/pull/1079#issuecomment-1497691993

Just want to look into it why is this happening.

Problem: why is it loaded when invoking `mvn help:help`

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No further details from [MNG-7761](https://issues.apache.org/jira/browse/MNG-7761?redirect=false)

Start with the behavior described in the linked comment on PR #1079 and reproduce it with `mvn help:help`. Trace Maven's plugin loading path to determine why the site plugin is loaded, then document the cause and the expected behavior or next action.

Written by the indexing model from the issue text.