make find_stdlib_deps more resiliant when weird things are inside the package folder

[KristofferC]

Fixes #314

[codecov-io]

Codecov Report

Merging #316 into master will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
- Coverage   88.78%   88.75%   -0.04%     
==========================================
  Files          16       16              
  Lines        2747     2748       +1     
==========================================
  Hits         2439     2439              
- Misses        308      309       +1

Impacted Files	Coverage Δ
src/Operations.jl	93.44% <66.66%> (-0.16%)	⬇️

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