Fix mod init

[ChloePlanet]

Tested under these situations:

• inside GOPATH
  • under root dir: --root flag needs to be set
  • under root subdir
• outside GOPATH

[codecov[bot]]

Codecov Report

Merging #1045 into master will increase coverage by 1.12%. The diff coverage is 83.87%.

@@            Coverage Diff             @@
##           master    #1045      +/-   ##
==========================================
+ Coverage   83.35%   84.48%   +1.12%     
==========================================
  Files          73       73              
  Lines       10609    10523      -86     
==========================================
+ Hits         8843     8890      +47     
+ Misses       1432     1325     -107     
+ Partials      334      308      -26     

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	56.96% <48.07%> (-12.85%)	:arrow_down:
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/parse/listener_impl.go	88.75% <76.34%> (-0.76%)	:arrow_down:
pkg/parse/fs.go	90.90% <80.00%> (-9.10%)	:arrow_down:
...g/mermaid/integrationdiagram/integrationdiagram.go	83.19% <82.45%> (-1.19%)	:arrow_down:
pkg/importer/openapi.go	86.12% <85.45%> (-3.32%)	:arrow_down:
... and 29 more