fix[lang]: fix importing of flag types

[cyberthirst]

What I did

• fix import of flag types: flag types not properly imported via modules #3827
• modify grammar to allow for nested import types

How I did it

How to verify it

• wrote a new test for flag imports: test_flag_imports.py

Commit message

fix imports for flag types. this is a missed case from 8ccacb3f47f.

this commit adds flags into the module data and threads them through
codegen.

Description for the changelog

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.38%. Comparing base (e589278) to head (b180e2f).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3871      +/-   ##
==========================================
+ Coverage   86.33%   86.38%   +0.04%     
==========================================
  Files          92       92              
  Lines       14010    14017       +7     
  Branches     3077     3079       +2     
==========================================
+ Hits        12096    12108      +12     
+ Misses       1484     1480       -4     
+ Partials      430      429       -1     

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[charles-cooper]

thanks!