fix: don't crash on a licence that isn't an SPDX identifier

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
Every command that uses the extension scaffolder (infra, upgrade, …) loads the SPDX text for the license in composer.json. Anything other than a known SPDX ID or exactly proprietary made it throw Cannot find module, e.g. "© Glowing Blue AG" or "PROPRIETARY". Licences are now matched against the SPDX list ignoring case, and any other value gives an empty licence text.

Reviewers should focus on:

• A licence given as an array also gets no licence text, rather than using the first entry.

Screenshot
N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered? (N/A)
• For core PRs, does this need to be in core, or could it be in an extension? (N/A)
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (The built CLI ran infra editorConfig on a copy of an extension with a free-form licence.)
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test). (N/A)
• Backend changes: tests have been added, or are not appropriate here. (N/A)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.