Aggregate raw coverage data, not XML reports

[gslowikowski]

When generating aggregated reports in multi-module builds, coverage data were read from per-module XML reports - scoverage.xml files.

These reports contain the same data as internal raw data files: scoverage.coverage and scoverage.measurements.*.

Coverage data for aggregated reports will be read from raw data files. This has some advantages:

• generating per-module reports (coverageReport) is not required prior to generating aggregated report (coverageAggregate); this is the most important advantage,
• reading coverage data from plain text files is usually (from my experience - always) faster than parsing XML files
• fixes small bug in aggregated scoverage-debug.xml report, generated when coverageOutputDebug is set to true (statement element's value as well as symbol and tree attributes were not filled)