Minor improvements to importer pkg

[cuminandpaprika]

This PR covers mostly minor miscellaneous fixes in the importer pkg. It aims to improve the readability of the importer package and also fixes some test files in preparation for a future PR.

Changes Proposed:

• Fix incorrect test files
• Add nil check to openapi importer
• Renames and moves some functions for readability

Checklist:

• [x] Added related tests
• [x] Made corresponding changes to the documentation

[codecov[bot]]

Codecov Report

Merging #898 into master will increase coverage by 0.01%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #898      +/-   ##
==========================================
+ Coverage   83.35%   83.36%   +0.01%     
==========================================
  Files          73       73              
  Lines       10609    10625      +16     
==========================================
+ Hits         8843     8858      +15     
- Misses       1432     1433       +1     
  Partials      334      334              

Impacted Files	Coverage Δ
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	:arrow_up:
pkg/importer/openapi.go	88.95% <80.00%> (-0.50%)	:arrow_down:
pkg/importer/utils.go	88.23% <100.00%> (+0.35%)	:arrow_up:
...g/mermaid/integrationdiagram/integrationdiagram.go	86.66% <100.00%> (+2.29%)	:arrow_up:
pkg/mod/module.go	67.56% <100.00%> (+1.85%)	:arrow_up: