SignTool error logs: Display list of packages that a file is contained in

[JohnTortugo]

Closes: #1272

Goal: when printing error messages also print all the packages that a file appears in so that the user can easily locate in which package(s) the file was found.

Changes:

• SignToolTests.cs: patched tests output to include the relative path of the files.
• Configuration.cs:
  • keep track of the packages that each file appears in;
  • keep tracks of which files produced an specific error.
  • print errors at the end.

These changes were tested here: https://dnceng.visualstudio.com/internal/_build/results?buildId=47746&view=logs where you can also see the format of the output.

[JohnTortugo]

Please see on this link how the output will look like: https://dnceng.visualstudio.com/internal/_build/results?buildId=47746&view=logs

[natemcmaster]

Thanks for fixing! This will help a lot in diagnosing signing misconfiguration.

I made one suggestion, but otherwise this looks good to me.