registration: Improve license-name validation

registration: Improve license-name validation

Our hardcoded enum list in the extension.json schema for license-name
values was incomplete and did not cover the full SPDX license identifier
specification, which includes things like "AND" for specifying multiple
licenses.

Composer already has solid code in a library to do this validation, so
let's use it! This updates both the validateRegistrationFile.php and
ExtensionJsonValidationTest.php to use the composer/spdx-licenses
library (a new development dependency) to ensure the license-name field
is a valid SPDX identifier.

Also fix a silly typo in the validateRegistrationFile script which
prevented it from running, and use ::class so it will be easier to
detect typos like that in the future.

Bug: T146862
Change-Id: Ibb8973ed7950ae81c90558f9630f73746b2aff2c