Remove global fetcher cache

[edolstra]

Motivation

This removes the global fetcher cache object (the getCache() singleton) and moves the cache into fetchers::Settings. This is a bit of an abuse of fetchers::Settings but I didn't want to add another class to pass around everywhere.

Context

This is cherry-picked from DeterminateSystems#49, which adds a builtin:fetch-tree builtin builder. Since builtin builders run in a forked child process, we cannot use any of the parent's global state (like the fetcher cache's SQLite database) since that's invalid in the child (e.g. it refers to file handles that don't exist in the child). Thus we use a new fetchers::Settings object to avoid touching the parent's fetcher cache.

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[Ericson2314]

@edolstra Thank you for joining me in the war against global variables! :)

[roberth]

This is a bit of an abuse of fetchers::Settings

This has been done before, and I'm not too worried about it.
Settings could be split if we have a need for it. We could have Context containing Settings or something, but at this point I feel like that would just be annoying. Maybe in the future when we have a reason to do that.