SPDX 2->3 conversion of excludedFiles in PackageVerificationCode

[armintaenzertng]

In SPDX2 we could express a list of excluded files, how is this possible in SPDX3?

Gary's comment from #74:

The idea is we would have a separate verification implementation for packages that would include this information. The work for this hasn't been done.

This has been discussed in the tech team call on 2023-02-28. It is not clear yet whether PackageVerificationCode will be carried over to SPDX3.

[goneall]

The current proposal is to completely replace verification code with a different algorithm.

There still is a migration issue in what we do with the old verification code. It is, however, optional in SPDX 3.

[goneall]

Fixed with PR #345