Refactor importer to use interface

[cuminandpaprika]

This PR aims to make importers more generic in the way they are called. It also simplifies the way formats are detected and more rigorously checks the file format.

Changes proposed in this pull request:

• Add GuessFileType function to handle all conerns related to format detection
• Add Importer interface that allows importers to be referenced in an uncoupled way
• Add importer.Factory function which takes in a filename, and a file and returns an Importer
• Refactors existing code to use the new interface
• Add tests using sample full OpenAPI specs from the official repo

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #899 into master will increase coverage by 0.07%. The diff coverage is 82.02%.

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   83.35%   83.42%   +0.07%     
==========================================
  Files          73       75       +2     
  Lines       10609    10710     +101     
==========================================
+ Hits         8843     8935      +92     
- Misses       1432     1440       +8     
- Partials      334      335       +1     

Impacted Files	Coverage Δ
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	:arrow_up:
pkg/parse/parse.go	83.47% <69.56%> (-0.50%)	:arrow_down:
pkg/importer/swagger.go	76.00% <73.33%> (-3.60%)	:arrow_down:
pkg/importer/openapi.go	91.62% <83.95%> (+2.17%)	:arrow_up:
pkg/importer/formats.go	90.90% <90.90%> (ø)
pkg/importer/xsd.go	91.01% <95.23%> (+0.64%)	:arrow_up:
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:
... and 4 more