[MRESOLVER-589] ArtifactDescriptorResult should be immutable

**[Tamas Cservenak](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=cstamas)** opened **[MRESOLVER-589](https://issues.apache.org/jira/browse/MRESOLVER-589?redirect=false)** and commented

And most probably heavily cached, as on collection of large projects, this leads to insane memory requirements. MRESOLVER-587 and MRESOLVER-586 adds some relief, but still the biggest offender is depMgt that on projects usually get inherited from parent (and can have non trivial size) and is copied over and over again...

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**Issue Links:**
- [MNG-8242](https://issues.apache.org/jira/browse/MNG-8242) Cache flattened parents during model building
(_**"is fixed by"**_)
- [MRESOLVER-586](https://issues.apache.org/jira/browse/MRESOLVER-586) In typical setups, DefaultArtifact copies the same maps over and over again

- [MRESOLVER-587](https://issues.apache.org/jira/browse/MRESOLVER-587) Memory consumption improvements

Start by locating ArtifactDescriptorResult in the Java sources and tracing its constructors, fields, callers, and related tests. Review the linked MRESOLVER-586 and MRESOLVER-587 issues for surrounding memory concerns; done means the result is immutable without breaking existing resolver behavior, with the relevant test suite passing.

Written by the indexing model from the issue text.