Make sure the sources are correct when information is aggregated

[carmenbianca]

Since #778, the source information is incorrect. This was flagged before here.

Also fixes #753

[carmenbianca]

On second thought, I think we should keep this method (+ its tests)

[linozen]

Yes, I would also be in favor of leaving it in I think.

But otherwise, everything looks good to me. So feel free to merge when it suits you.

[linozen]

One small thing I just noticed that should be included here perhaps is an explanation of how this kind of breaks JSON as in the JSON output there really is only one source given.

Or would you want to do that in a separate PR or with the next release?

[carmenbianca]

One small thing I just noticed that should be included here perhaps is an explanation of how this kind of breaks JSON as in the JSON output there really is only one source given.

Au contraire :) Here's a part of the output when running reuse lint --json on this repository:

    {
      "path": "po/nl.po",
      "copyrights": [
        {
          "value": "2023 Free Software Foundation Europe e.V. <https://fsfe.org>",
          "source": ".reuse/dep5",
          "source_type": "dep5"
        },
        {
          "value": "SPDX-FileCopyrightText: 2018, 2020 Carmen Bianca Bakker",
          "source": "po/nl.po",
          "source_type": "file-header"
        },
        {
          "value": "SPDX-FileCopyrightText: 2017, 2020 Andr\u00e9 Ockers",
          "source": "po/nl.po",
          "source_type": "file-header"
        }
      ],
      "spdx_expressions": [
        {
          "value": "GPL-3.0-or-later",
          "source": ".reuse/dep5",
          "source_type": "dep5"
        },
        {
          "value": "GPL-3.0-or-later",
          "source": "po/nl.po",
          "source_type": "file-header"
        }
      ]
    },

This PR fixes exactly what you just described :D

[linozen]

This PR fixes exactly what you just described :D

Amazing! Then this is ready for a merge IMHO.

[carmenbianca]

Let's first undo the removal of the method I flagged, and then let's merge :D