Fix importer format detection for OpenAPI and Swagger

[cuminandpaprika]

Fixes a bug with file format detection when the below is used, which is valid openapi

openapi: 3.0.0

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #925 into master will decrease coverage by 0.02%. The diff coverage is 83.38%.

@@            Coverage Diff             @@
##           master     #925      +/-   ##
==========================================
- Coverage   83.35%   83.33%   -0.03%     
==========================================
  Files          73       75       +2     
  Lines       10609    10744     +135     
==========================================
+ Hits         8843     8953     +110     
- Misses       1432     1451      +19     
- Partials      334      340       +6     

Impacted Files	Coverage Δ
pkg/parse/listener_impl.go	89.59% <62.50%> (+0.08%)	:arrow_up:
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/mod/module.go	66.66% <72.72%> (+0.95%)	:arrow_up:
pkg/importer/swagger.go	76.00% <73.33%> (-3.60%)	:arrow_down:
pkg/pbutil/output.go	76.00% <75.00%> (-0.48%)	:arrow_down:
pkg/importer/formats.go	77.27% <77.27%> (ø)
pkg/importer/openapi.go	89.42% <88.98%> (-0.02%)	:arrow_down:
pkg/importer/xsd.go	91.01% <95.23%> (+0.64%)	:arrow_up:
... and 5 more