feat: cache npm results in memory per-run

We currently use got’s inbuilt caching for npmjs, however it still results in a roundtrip/304 even when cached. The when using presets is that a package like renovate-config-default gets looked up 10+ times unnecessarily. With this feature we assume that npm lookups don’t change for the duration of a run, and even if they did then we’d prefer the reliability of using the first lookup.

Closes #1389