fix #882, add better validator

[nofun97]

Fixes #882 .

Changes proposed in this pull request:

sysl validate command now has linter warnings, it checks the following:

1. Return statements that does not use ok
2. Check that endpoint exists in a call statement
3. Check for case-sensitive app definition

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #910 into master will increase coverage by 1.10%. The diff coverage is 87.08%.

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
+ Coverage   83.35%   84.45%   +1.10%     
==========================================
  Files          73       75       +2     
  Lines       10609    10482     -127     
==========================================
+ Hits         8843     8853      +10     
+ Misses       1432     1318     -114     
+ Partials      334      311      -23     

Impacted Files	Coverage Δ
pkg/pbutil/output.go	76.00% <ø> (-0.48%)	:arrow_down:
pkg/syslwrapper/test_helper.go	57.89% <ø> (-1.42%)	:arrow_down:
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	:arrow_up:
pkg/mod/module.go	66.66% <72.72%> (+0.95%)	:arrow_up:
pkg/parse/parse.go	85.34% <73.07%> (+1.37%)	:arrow_up:
pkg/importer/openapi.go	83.59% <82.67%> (-5.86%)	:arrow_down:
pkg/parse/listener_impl.go	89.62% <86.48%> (+0.11%)	:arrow_up:
pkg/importer/swagger.go	81.48% <88.88%> (+1.88%)	:arrow_up:
pkg/parse/linter.go	90.84% <90.84%> (ø)
... and 20 more

[nofun97]

@anz-gordonj7

1. Move the linter functions to (at least) their own file, listener_impl.go is way too big already.

👍 will move it to its own file

2. why make these linter warnings instead of validation errors?

The plan is later to make sysl validate fails too but it seems that's going to be a big change. The sysl modules are loaded before executing the command so it's going to be tricky. So I thought this PR should be just about detecting the problems and have another PR that will make sysl validate fail.