Cover embed source files

[Blebowski]

This MR copies each source file to a stand-alone HTML file when generating code coverage report. The generated HTML file is then referenced from the "File" in the coverage hierarchy header. This is usefull when e.g. a regression is run in CI, and code coverage report is extracted in artifacts. After downloading the artifacts, the reference to the source files remains valid.

To do this, I needed to track location of the underlying "unit" for the scope, not the "instance". It affects the XML file output, but I guess this is no big deal since this format is subject to change. The issue909 also now gives more relevant output since the source file for instance itself can't be found.