Improve source paths in XML report

[DirkMahler]

The source paths in the XML report shall be relative to the project root (or Git/Subversion/etc root folder) as this normally indicates the path which is evaluated by tools like SonarQube or Teamscale to pin findings to files.

This is also a pre-requisite to support writing SARIF-reports.

[DirkMahler]

The items outlined here should be considered during implementation.