Add imports only test, fix instrumenter

[cgewecke]

#429

[codecov-io]

Codecov Report

Merging #450 into buidler-plugin will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           buidler-plugin     #450      +/-   ##
==================================================
+ Coverage           99.22%   99.22%   +<.01%     
==================================================
  Files                  20       20              
  Lines                 778      779       +1     
==================================================
+ Hits                  772      773       +1     
  Misses                  6        6

Impacted Files	Coverage Δ
lib/instrumenter.js	100% <100%> (ø)	⬆️

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