Add `SPDX-License-Identifier` as alised for "license" module header tokens

C.f. SPDX 2.1 - Appendix V
 https://spdx.org/spdx-specification-21-web-version#h.twlc0ztnng3b

    The tag should appear on its own line in the source file, generally as part of a comment.

    SPDX-License-Identifier: <SPDX License Expression>

Cherry-picked from #743