Use proper SPDX parsing

[noahnu]

Is your feature request related to a problem? Please describe.

The "loose" part of loose mode refers to lower confidence in parsing an accurate license. If we switch to a proper SPDX parser, we can improve the confidence, and enable this by default.

Describe the solution you'd like

Use something like https://github.com/jslicense/spdx-expression-parse.js

Describe alternatives you've considered

In-house solution.

Additional context

We have to be careful bringing in other dependencies as it's bundled into the plugin, especially as it seems SPDX parser doesn't get many updates.