[MNG-7991] refactor "aggregator" goal feature

**[Hervé Boutemy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hboutemy)** opened **[MNG-7991](https://issues.apache.org/jira/browse/MNG-7991?redirect=false)** and commented

aggregation was added in early Maven 2 stages with MNG-250, not really documented AFAIK, but highly visible through [@Mojo(aggregator=true) Mojo annotation](https://maven.apache.org/plugin-tools/maven-plugin-tools-annotations/index.html)

there are multiple shortcomings found over time:
* discrepancy between CLI and lifecycle use:
* CLI executes aggregator-marked goal **only on root module** (skips goal execution in sub-modules) and **after executing the lifecycle** to all modules
* lifecycle-bound goal executes goal on each and every module, during normal lifecycle run on each module: not even sure aggregator configuration changes anything

* forked lifecycle execution vs second execution in modules

* sometimes, instead of executing aggregate goal **before** building modules, it would be useful to execute aggregate **after** building modules (to aggregate some results built in each module)
notice that this looks also like the need for "execute at end" as found in install/deploy

we probably need to better document what aggregate currently does and define what should be the target: perhaps refactoring will require new mechanisms that we won't call aggregator, like MNG-5665...

edge case: aggregating reports for maven-site-plugin (launched by maven-reporting-exec, hence quite independent from associated reporting goals even if we expect some consistency between site reports and reporting goals)

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**Affects:** 4.x / Backlog

**Issue Links:**
- [MNG-2184](https://issues.apache.org/jira/browse/MNG-2184) Possible problem with `@aggregator` and forked lifecycles
(_**"contains"**_)
- [MNG-6336](https://issues.apache.org/jira/browse/MNG-6336) Aggregator Mojo should be executed only once even when part of the lifecycle
(_**"incorporates"**_)
- [MNG-250](https://issues.apache.org/jira/browse/MNG-250) make aggregation feasible
(_**"is caused by"**_)
- [MNG-5665](https://issues.apache.org/jira/browse/MNG-5665) Advanced Lifecycle Handling

- [MNG-7163](https://issues.apache.org/jira/browse/MNG-7163) Aggregating Mojo re-executes goals for child modules that are already executed

- [MNG-7672](https://issues.apache.org/jira/browse/MNG-7672) Aggregate goals executed in a submodule forks the whole reactor

- [MNG-2180](https://issues.apache.org/jira/browse/MNG-2180) Allow aggregation mojos to access plugin configurations

- [MNG-4504](https://issues.apache.org/jira/browse/MNG-4504) Disallowing all aggregator bindings to any lifecycle is too broad

- [MNG-8337](https://issues.apache.org/jira/browse/MNG-8337) aggregator goal called from CLI shows an additional module at end (= first module twice)

- [MNG-7900](https://issues.apache.org/jira/browse/MNG-7900) wrapper lifecycle in multi module project should be executed only in root module