Expose import resolver interface for cross-train/luis:build/qnamaker:build

[feich-ms]

fix #934 to expose import resolver interface for cross-train/luis:build/qnamaker:build so that composer can customize their file resolving system based on their import system.

[codecov-commenter]

Codecov Report

Merging #935 into master will decrease coverage by 2.82%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
- Coverage   72.12%   69.29%   -2.83%     
==========================================
  Files         175      175              
  Lines       10754     9051    -1703     
  Branches      900      887      -13     
==========================================
- Hits         7756     6272    -1484     
+ Misses       2669     2420     -249     
- Partials      329      359      +30     

Impacted Files	Coverage Δ
packages/lu/src/parser/cross-train/crossTrainer.js	95.21% <100.00%> (-1.26%)	⬇️
packages/lu/src/parser/lu/luMerger.js	77.89% <100.00%> (-3.77%)	⬇️
packages/lu/src/parser/lubuild/builder.ts	26.61% <100.00%> (+14.82%)	⬆️
packages/lu/src/parser/lufile/parseFileContents.js	81.21% <100.00%> (-3.63%)	⬇️
packages/lu/src/parser/qnabuild/builder.ts	34.12% <100.00%> (+14.33%)	⬆️
packages/lu/src/parser/utils/helpers.js	87.25% <100.00%> (-2.46%)	⬇️
packages/plugins/src/commands/plugins/install.ts	34.48% <0.00%> (-16.50%)	⬇️
...kages/qnamaker/src/commands/qnamaker/kb/publish.ts	85.71% <0.00%> (-14.29%)	⬇️
...aker/src/commands/qnamaker/endpointkeys/refresh.ts	85.71% <0.00%> (-14.29%)	⬇️
packages/config/src/commands/config/index.ts	75.00% <0.00%> (-13.89%)	⬇️
... and 103 more

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[boydc2014]

Looks good to me, @vishwacsena , @munozemilio can you help also check this one, this will help Composer to customize the interpretation of the import statements in lu or qna files. Like "import[a.lu]" or "import[b.qna]", composer can direct that import to a.en_us.lu or b.en_us.qna like that.