Rework `cache.Cache` to use `x/sync/singleflight`

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (877c587) 90.05% compared to head (250193d) 90.03%.

Files	Patch %	Lines
cache/generic.go	88.23%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #113 +/- ## ========================================== - Coverage 90.05% 90.03% -0.03% ========================================== Files 40 40 Lines 1940 1936 -4 ========================================== - Hits 1747 1743 -4 - Misses 153 155 +2 + Partials 40 38 -2 ```

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