Refactor importers to use an interface

[cuminandpaprika]

This is a fairly large PR that refactors the way that importers are called and removes the mode flag from the sysl import command. All file formats are now automatically detected. A warning message will now be presented if the flag is passed in.

This PR is best reviewed commit by commit.

Changes proposed in this pull request:

• Refactor sysl importers to use Importer interface
• Fix the way file detection is handled to be more specific

Checklist:

• Added related tests
• Made corresponding changes to the documentation

[codecov]

Codecov Report

Merging #904 into master will decrease coverage by 0.03%.
The diff coverage is 79.11%.

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
- Coverage   83.35%   83.32%   -0.04%     
==========================================
  Files          73       75       +2     
  Lines       10609    10738     +129     
==========================================
+ Hits         8843     8947     +104     
- Misses       1432     1451      +19     
- Partials      334      340       +6     

Impacted Files	Coverage Δ
pkg/parse/listener_impl.go	89.59% <62.50%> (+0.08%)	⬆️
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	⬆️
pkg/parse/parse.go	83.47% <69.56%> (-0.50%)	⬇️
pkg/mod/module.go	66.66% <72.72%> (+0.95%)	⬆️
pkg/importer/swagger.go	76.00% <73.33%> (-3.60%)	⬇️
pkg/pbutil/output.go	76.00% <75.00%> (-0.48%)	⬇️
pkg/importer/formats.go	77.27% <77.27%> (ø)
pkg/importer/openapi.go	90.00% <83.95%> (+0.55%)	⬆️
pkg/importer/xsd.go	91.01% <95.23%> (+0.64%)	⬆️
... and 5 more

[ChloePlanet]

LGTM