Update CDDL generation script to also read external definitions

[whimboo]

We have external definitions of WebDriver BiDi command and events like this on-exhaustive list shows:

https://w3c.github.io/webdriver-bidi/#external-specifications

Right now we do not yet read their CDDL defintions which means even if implemented in browsers clients will not know about in case they rely on auto-generated code.

We should probably update our cddl generation script to check external documents as well.

CC @jgraham @gsnedders

[gsnedders]

We should be careful to not accidentally change the definition of the Remote End Definition (or Local End Definition) if/when we do this; various things are matched against it, and if we explicitly include all external specifications in the definition, does that not oblige them to be implemented?

[whimboo]

The inclusion of any external spec would require an agreement by the Browser Testing and Tools group for sure. Further it would apply to any other CDDL changes afterward.

If we don't agree on that what else could we do to provide a CDDL export that also includes all the extension commands and events?

[tidoust]

I haven't had time to dig into it yet but I plan to work on adding CDDL extracts to Webref data in a not so distant future, see w3c/webref#1353.

There are also interesting considerations on the notion of CDDL modules in a recent IETF thread (initially focused on ABNF extracts), which could perhaps be used to create a consolidated view.