cache import path lookups results

[bhcleek]

Cache the results of import path lookups so that go list doesn't have to
be run every time linting is run or go#guru#Sameids() is run, because go
list can take a noticeably long time to return results and has to be
executed synchronously for all of its current use cases.

This further reduces the likelihood that characters will be rendered at
the cursor position when in normal mode.

[codecov-io]

Codecov Report

Merging #1713 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1713      +/-   ##
==========================================
- Coverage   19.57%   19.55%   -0.03%     
==========================================
  Files          57       57              
  Lines        4787     4792       +5     
==========================================
  Hits          937      937              
- Misses       3850     3855       +5

Flag	Coverage Δ
#nvim	14.94% <0%> (-0.02%)	⬇️
#vim74	17.13% <0%> (-0.02%)	⬇️
#vim80	18.48% <0%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
autoload/go/package.vim	0% <0%> (ø)	⬆️

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[fatih]

lgtm