Reduce `Debugger.*` friction between gatherers

[adamraine]

From https://github.com/GoogleChrome/lighthouse/pull/14717:

This PR has become a bit of a rabbit hole. While I do think it would be good to land this in the long term, it opens the door for some more complex challenges relating to scripts gathering and third parties.

Incoming brain dump...

Calling Debugger.enable when the domain is disabled will flush any Debugger.scriptParsed events that the session hadn't emitted yet. Since this only happens when the domain was in the disabled state, the events are emitted on the first invocation of Debugger.enable which previously happened in prepare.js as part of our navigation/timespan prework.

This PR moves that invocation into the DevtoolsLog gatherer, which moves the new first invocation of Debugger.enable into the Scripts gatherer. This doesn't mean anything for navigation mode, since the domain will always be enabled while we are on about:blank before the navigation starts. However, timespan mode can start on an already loaded page, which means there could be Debugger.scriptParsed events waiting to emit that will now be caught by the Scripts gatherer since the first invocation of Debugger.enable happens there. This means timespan mode will collect all scripts that are loaded during the timespan, as well as any scripts that were on the page at the start of the timespan. In theory this is a better way of collecting scripts for an arbitrary timespan however...

With the introduction of the new 3P system, we will throw an error if we encounter a url that doesn't exist in the network records. Now we are collecting scripts that weren't fetched during the timespan, which can cause this error to be thrown if we attempt to use a url from one of the pre-timespan scripts in say the valid-source-map audit:

https://github.com/GoogleChrome/lighthouse/blob/6de2ed25cdf2da46ed1bc261486d638f7a5adc49/core/audits/valid-source-maps.js#L56-L62

In the future, I imagine we will classify the urls in Scripts but for now I think the easiest workaround is to call Debugger.enable in the navigation/timespan prework so all the Debugger.scriptParsed events are flushed before gathering starts. Or we could just do https://github.com/GoogleChrome/lighthouse/pull/14716 w/ @brendankenny's suggestions.

To try and summarize:

1. We should start using separate sessions for different gatherers/tasks that require the debugger for different things.
2. I wanted to move the async stack handlers to the DevTools log gatherer without worrying about how it affects other gatherers like Scripts
3. Scripts can then collect scripts that were on the page at the start of timespan mode, however...
4. Our 3P system will need to support urls that are not seen in any network request. This probably means we need to feed Scripts into EntityClassification