Improve coverage statistics logging

[andrei-epure-sonarsource]

We currently (8.15) print statistics about the code coverage import:

2020-12-10T16:56:42.8117691Z INFO: Coverage Report Statistics: 923 files, 923 main files, 923 main files with coverage, 0 test files, 0 project excluded files, 0 other language files.

Another nice to have is to improve the statistics:

• count skipped files (due to indexing/language) and print them in the statistics
• count files with _ in the path and add the numbers to the existing statistics line (related Add support for Deterministic Source Paths (VS Coverage format) #3817 )

[andrei-epure-sonarsource]

Also make sure the paths from the logs use the OS File.Separator (see this comment)