feat(nuxt): supports automatic import of stores from extend layers

[wzc520pyfm]

When I install pinia in a nuxt layer and create stores folder, and then I extend this layer in another nuxt application, I find that the stores are not automatically imported.

What this PR does:

When addImportsDir, consider the nuxt layers also.

Refer:

Pinia autoImports for nuxt

Nuxt layers

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commit: 3f4442f

[posva]

I noticed there is also https://github.com/vuejs/pinia/pull/2757, so let's use that one instead. Thanks for the PR!

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 89.34%. Comparing base (6e90153) to head (3f4442f). :warning: Report is 49 commits behind head on v2.

Additional details and impacted files

@@           Coverage Diff           @@
##               v2    #2828   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files          14       14           
  Lines        1323     1323           
  Branches      218      219    +1     
=======================================
  Hits         1182     1182           
  Misses        140      140           
  Partials        1        1           

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