fix: fail when searchDir contains no package

[easonlin404]

fix #432

[codecov-io]

Codecov Report

Merging #434 into master will decrease coverage by 0.11%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   81.95%   81.83%   -0.12%     
==========================================
  Files           7        7              
  Lines        1441     1448       +7     
==========================================
+ Hits         1181     1185       +4     
- Misses        156      157       +1     
- Partials      104      106       +2

Impacted Files	Coverage Δ
parser.go	79.36% <60%> (-0.18%)	⬇️

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