[MDEP-485] unpack-dependencies should be able to use subdirectories per classifier

**[Dirk Rudolph](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=diru)** opened **[MDEP-485](https://issues.apache.org/jira/browse/MDEP-485?redirect=false)** and commented

When using classifiers to build environment specific artefacts, aggregating them to for example a single deployment unit (in my case its an rpm file, using the rpm plugin) would be easier when there would be a configuration option to unpack all dependencies to a subdirectory per classifier.

```

true

```

In this case we could easily unpack all dependencies build for development environment to a single place. Adding a new artefact would require adding a dependency and **not** changing the plugin configuration for the build of the deployment unit.

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**Affects:** 2.10

The affected entry point is the unpack-dependencies goal; first trace how it handles dependency classifiers and how its configuration is documented. Add the requested useSubDirectoryPerClassifier option and verify that dependencies with different classifiers unpack into separate classifier-based directories without changing existing behavior.

Written by the indexing model from the issue text.