cmd/watchflakes/internal/cache: very simple file system cache
cmd/watchflakes/internal/cache: very simple file system cache

Needs tests.

Change-Id: Ia03bcdb97220fcfe55125413eea2e3683f12494b
Reviewed-on: https://go-review.googlesource.com/c/build/+/432401
Auto-Submit: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build