Use different SPDX license source

[isoos]

The .txt license files may contain the optional texts of the licenses. For most license it also contained the END OF TERMS AND CONDITIONS text, and we were using it to separate the main license part from the optional one. However, a recent update of the license data in the LGPL-3.0 does not have this distinction, and the optional part is not separated by a reasonable string from the main:
https://github.com/spdx/license-list-data/blame/master/text/LGPL-3.0-only.txt#L72-L77

This blocks the update of the license data files.

Instead of using the txt files, we should rather use the source XMLs, (or maybe the generated JSONs), which does have a clear separation of the optional parts.

[sigurdm]

Seems still relevant