eslint-plugin-import has some useful rules to ensure that import files, names and other items are valid.

We already have the module in-tree, though it isn't actually turned on anywhere afaict (newtab attempts include it, but doesn't actually use any rules from it - I'll file a separate bug for that later).

The first version of this that I'm attaching turns on a lot of rules that I think are relevant and useful. For now, we turn off the no-unresolved (i.e. the file can't be found) for system mjs files and for some directories which have their own package.json. These each have their own issues, which I'll file follow-up bugs for.

A few comments about particular things I've not turned on at this stage:

• no-unused-modules
  • This does a separate parse of all mjs files in the tree, causing big performance lags and doesn't seem worth having it on all the time (at least yet, I think we should explore it in future, once we have more files transitioned across).
• no-cycle
  • This is also an expensive rule. Like no-unused-modules, I think we could explore this later.
• no-internal-modules
  • I think we probably break this quite extensively at the moment.

Depends on D148773

https://hg.mozilla.org/mozilla-central/rev/7e5b24990d99