fix(utils): include routes with children for generation

[farnabaz]

flatRoutes function was skip all parent routes that have children. Parent routes was not generates in static generation, to Take a look at #4562 to find more about the issue.
close #4562 & #4982

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #7761 into dev will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #7761      +/-   ##
==========================================
+ Coverage   68.83%   68.85%   +0.02%     
==========================================
  Files          90       90              
  Lines        3821     3824       +3     
  Branches     1034     1034              
==========================================
+ Hits         2630     2633       +3     
  Misses        968      968              
  Partials      223      223              

Flag	Coverage Δ
#unittests	68.85% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/utils/src/route.js	96.73% <100.00%> (+0.06%)	⬆️

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[farnabaz]

@pi0 I think one of previous tests is wrong, Look at these lines. This route should create three pages (exact behavior as vue-router)

https://github.com/nuxt/nuxt.js/pull/7761/files#diff-f20ead15adb68c21668ec1154f3ba2a2L44-R56

[pi0]

LGTM. @clarkdo @Atinux Would you please also confirm new behavior? (/foo is being added)

[Atinux]

Nice work!