fix mod.Retrieve

[ChloePlanet]

Changes proposed in this pull request:

• Find the existing module in go modules mode and there's no version tag

[codecov[bot]]

Codecov Report

Merging #1036 into master will increase coverage by 1.02%. The diff coverage is 82.34%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
+ Coverage   83.35%   84.37%   +1.02%     
==========================================
  Files          73       77       +4     
  Lines       10609    10714     +105     
==========================================
+ Hits         8843     9040     +197     
+ Misses       1432     1354      -78     
+ Partials      334      320      -14     

Impacted Files	Coverage Δ
pkg/database/db_test_utils.go	83.33% <0.00%> (ø)
pkg/diagrams/plantuml.go	0.00% <0.00%> (ø)
pkg/mod/fs.go	67.16% <51.61%> (-2.65%)	:arrow_down:
pkg/mod/github.go	55.85% <55.85%> (ø)
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mermaid/datamodeldiagram/datamodeldiagram.go	61.90% <68.42%> (+3.33%)	:arrow_up:
pkg/parse/parse.go	85.01% <71.87%> (+1.03%)	:arrow_up:
pkg/mod/gomodules.go	73.41% <77.77%> (ø)
pkg/mod/module.go	82.85% <79.31%> (+17.14%)	:arrow_up:
pkg/parse/fs.go	90.90% <80.00%> (-9.10%)	:arrow_down:
... and 32 more