teach filecache to make its dir

[jbowes]

No description provided.

[codecov]

Codecov Report

Merging #7 (5fe4295) into main (96a1f35) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   93.22%   93.33%   +0.11%     
==========================================
  Files           3        3              
  Lines         118      120       +2     
==========================================
+ Hits          110      112       +2     
  Misses          4        4              
  Partials        4        4              

Impacted Files	Coverage Δ
impl/file.go	100.00% <100.00%> (ø)

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