analyzer.go: add support for nested go modules

[kirbyquerby]

This change adds support for analyzing projects with more than one go.mod. This will let us run this tool on https://github.com/cosmos/cosmos-sdk

Updates cosmos/cosmos-sdk#10572
Related: securego/gosec#501

[codecov-commenter]

Codecov Report

Merging #23 (161133d) into master (d50c2b8) will decrease coverage by 0.31%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   71.05%   70.74%   -0.32%     
==========================================
  Files           9        9              
  Lines         570      581      +11     
==========================================
+ Hits          405      411       +6     
- Misses        143      146       +3     
- Partials       22       24       +2     

Impacted Files	Coverage Δ
analyzer.go	89.02% <66.66%> (-2.48%)	⬇️

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[odeke-em]

LGTM, thank you @kirbyquerby!