completions/zsh: Improve caching behaviour

[okdana]

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This addresses #5839 and a few related issues:

• Eliminate reliance on external utility calls for cache invalidation
• Update cache policy to also invalidate if cache file is >=2 weeks old or if tap indexes are non-existent
• Do not override any cache policy the user might already have set
• Handle default cache policy determination as early as possible to ensure that a default is set appropriately for all helpers (this will mostly be useful if other helpers need caching in the future)

It's notoriously hard to test zsh completion functions, so if anyone feels like double-checking my work that would be pretty cool.

I noticed some other issues with this function too; i'll try to get around to making separate PRs for them

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @okdana!