SPDX specifies that license identifiers (unlike the 'AND', 'OR', and
'WITH' operators) are matched case-insensitively.
* guix/import/utils.scm (%spdx-license-identifiers): New variable.
(spdx-string->license): Search in '%spdx-license-identifiers' using
'string-ci=?'.
* tests/import-utils.scm ("spdx-string->license"): New test.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>