handles tuple diagram gen with sequence of primitives fields

[nofun97]

Initially, tuple type whose fields are sequence of primitives generate the wrong plantuml diagram. This PR fixes that.

For example:

App:
    !type VeryComplex:
        complex <: sequence of string

Before	After

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #947 into master will increase coverage by 0.78%. The diff coverage is 83.88%.

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   83.35%   84.14%   +0.78%     
==========================================
  Files          73       74       +1     
  Lines       10609    10309     -300     
==========================================
- Hits         8843     8674     -169     
+ Misses       1432     1322     -110     
+ Partials      334      313      -21     

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/parse/listener_impl.go	89.50% <61.53%> (+<0.01%)	: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/formats.go	77.27% <77.27%> (ø)
pkg/importer/openapi.go	81.73% <80.71%> (-7.72%)	:arrow_down:
pkg/importer/swagger.go	81.48% <88.88%> (+1.88%)	:arrow_up:
... and 16 more