Use gex internally

[izumin5210]

for #83

manage executable development tools with gex

[codecov-io]

Codecov Report

Merging #88 into v0.3.0 will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           v0.3.0      #88      +/-   ##
==========================================
+ Coverage   45.32%   45.36%   +0.04%     
==========================================
  Files          30       30              
  Lines        1112     1111       -1     
==========================================
  Hits          504      504              
+ Misses        574      573       -1     
  Partials       34       34

Impacted Files	Coverage Δ
pkg/grapicmd/cmd/generate.go	0% <0%> (ø)	⬆️
pkg/grapicmd/protoc/plugin.go	83.33% <0%> (ø)	⬆️
pkg/grapicmd/cmd/init.go	0% <0%> (ø)	⬆️
pkg/grapicmd/cmd/protoc.go	0% <0%> (ø)	⬆️

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