builder: Include font build dependencies in meta table

I'm contemplating storing the build dependencies in every font's "meta" table. We have a situation where many designers haven't pinned the dependencies so we don't truly know what was used. By genning this table at build time, we can be certain of the dependencies.

I've just checked pyftsubset and it'll drop this table by default, which seems like something we'd want to do for this use case. I'd love the fonts in the google/fonts repo to have this table. I don't mind if the fonts on [fonts.google.com](http://fonts.google.com/) don't have it.

I've talked about this idea with Cosimo but I'm interested in hearing from everyone else.

This idea has already been discussed in 2017, https://github.com/googlefonts/fontmake/issues/251.

In the next post, I'll workout how big this table will be using Cosimo's dependency lister, https://gist.github.com/anthrotype/531a425c8a0ba5ee975bc2ec8add7b82