remove_files_for_content library

1. pcov package
2. documentation
3. remove_files_for_content.dart

pcov package

1. Libraries
2. create_file_references
3. get_dart_files
4. get_package_name
5. get_pcov_configuration
6. pcov_configuration
7. remove_files_for_content
8. remove_files_for_names
9. run_test_command