Make it easier to get the SPDX license expression for individual matches

Starting with output format version 3, there is `detected_license_expression_spdx`, but this seems to hold the combined expression for *all* matches in a file. However, in [ORT](https://github.com/oss-review-toolkit/ort) we're interested in the SPDX expressions for individual matches that come with start / end line numbers.

Matches do have `license_expression`, but these use ScanCode-specific keys instead of SPDX IDs for licenses. Before output format version 3, we were able to work around this by replacing `key` values with `spdx_license_key` values in expressions, but in output format version 3 `spdx_license_key` is not available anymore.

TL;DR if `matches` would have `license_expression_spdx` in addition to `license_expression` we'd be happy 😉