fixes attribute scope issues in type

[nofun97]

Initially attributes in the higher scope replaces attributes that are in lower scope.
This shouldn't happen.

For example:

App:
    !type attr:
        @a = 1
        i <: string:
            @a = 2

The resulted sysl module would replace the a attribute under the
field i with the value 1 which is higher in the scope.

This commit ensures that if attr values in lower scopes can not
be replaced by values in the higher scope.

Checklist:

• Added related tests
• Made corresponding changes to the documentation

[codecov]

Codecov Report

Merging #939 into master will increase coverage by 0.67%.
The diff coverage is 83.83%.

@@            Coverage Diff             @@
##           master     #939      +/-   ##
==========================================
+ Coverage   83.35%   84.03%   +0.67%     
==========================================
  Files          73       74       +1     
  Lines       10609    10302     -307     
==========================================
- Hits         8843     8657     -186     
+ Misses       1432     1335      -97     
+ Partials      334      310      -24     

Impacted Files	Coverage Δ
pkg/pbutil/output.go	76.00% <ø> (-0.48%)	⬇️
pkg/parse/listener_impl.go	89.50% <61.53%> (+<0.01%)	⬆️
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/formats.go	77.27% <77.27%> (ø)
pkg/importer/openapi.go	81.73% <80.71%> (-7.72%)	⬇️
pkg/importer/swagger.go	81.48% <88.88%> (+1.88%)	⬆️
pkg/importer/xsd.go	91.01% <95.23%> (+0.64%)	⬆️
... and 13 more

[cuminandpaprika]

LGTM, we might want to document the scope overwriting behaviour, since I had no idea Sysl did that at all?