[MDEP-551] recursive dep:tree format dot just appends

**[Paul Hammant](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=paul)** opened **[MDEP-551](https://issues.apache.org/jira/browse/MDEP-551?redirect=false)** and commented

For a multi-module build..

```
mvn dependency:tree -DappendOutput=true -DoutputType=dot -DoutputFile=/path/to/file.dot
```

... just appends multiple dot files together rather than composes a cohesive large dot (graphviz) file.

No graphviz capable tool can load the output :(

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No further details from [MDEP-551](https://issues.apache.org/jira/browse/MDEP-551?redirect=false)

Start with the dependency:tree goal and its handling of -DappendOutput=true, -DoutputType=dot, and -DoutputFile. Trace how output from a multi-module build is written, then verify that the resulting file is one cohesive Graphviz dot file that a graphviz-capable tool can load.

Written by the indexing model from the issue text.