[MCHECKSTYLE-370] Make file names relative

**[Dustin Singleton](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=singleton06)** opened **[MCHECKSTYLE-370](https://issues.apache.org/jira/browse/MCHECKSTYLE-370?redirect=false)** and commented

It would be nice if the checkstyle plugin can be configured to transform the file paths from absolute paths to relative paths in the resulting xml report.

I'm willing to work on this, I just wanted to get approval that this feature request would be accepted if I were to create a pull request.

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**Issue Links:**
- [MCHECKSTYLE-385](https://issues.apache.org/jira/browse/MCHECKSTYLE-385) Violation should be a value class
(_**"requires"**_)

Start by tracing how the Maven Checkstyle Plugin produces its resulting XML report and where its configuration is handled. The feature is done when users can configure absolute file paths to be emitted as relative paths in the XML report, with the intended relative base and behavior verified.

Written by the indexing model from the issue text.