Previously, '+' was supported only via special cases for deprecated
GNU identifiers like 'GPL-N+'. This commit adds support
for other uses of '+', such as 'AFL-2.0+' and 'LPPL-1.0+'.
Strictly speaking, '+' is an operator, not part of the SPDX license
identifier, but it is useful to handle it here.
* guix/import/utils.scm (spdx-string->license): Support '+' operator.
* tests/import-utils.scm ("spdx-string->license"): Test it.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
