Allow any gatherer to be a dependency

see https://github.com/GoogleChrome/lighthouse/discussions/15367#discussioncomment-6694261

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

We could either:
1 - explicitly set `symbol` in the meta + as a static property of every gatherer
2 - just use a string (the name of the artifact) - not good for reason mentioned in original comment
3 - define a `symbol` getter on the base gatherer:

```js
static get symbol() {
return Symbol.for(this.name);
}
```
4 - just use a reference to the class directly?

Start with the linked GitHub Discussion and the MDN Symbol reference to understand the dependency collision problem and the proposed alternatives. Then trace how gatherers currently declare and consume dependencies; done means a documented, accepted approach allows any gatherer to be used as a dependency without ambiguity.

Written by the indexing model from the issue text.