fix multiturn id mismatching issue

[feich-ms]

fix #886 to resolve the issue that id mismatched in multi turn prompts when merging multiple qna files. The fix is to use lu/qna merger to merge multiple qna files and resolve the follow up ids as a whole instead of resolving each file first and then directly merging them. It also fixed an issue in crosstrain that will have negetive impact for the id matching flow, that means I added the source > !# @qna.pair.source = crosstrained for all crosstrained DeferToRecognizer_LUIS_{dialogName} QA pair. Then I filtered out such pairs when resolving the multi turn prompt ids to make sure the follow up turns always point to the right original QA pairs. The reason I did this is that, some orginial questions will be added to crosstrained DeferToRecognizer_LUIS_{dialogName}, when resolving the follow up prompt ids, it may point to crosstrained DeferToRecognizer_LUIS_{dialogName} QA pair. This is unreasonable.

Unit test is added here

[codecov-commenter]

Codecov Report

Merging #902 into master will increase coverage by 0.66%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
+ Coverage   57.94%   58.61%   +0.66%     
==========================================
  Files         218      218              
  Lines       15365    15366       +1     
  Branches     2146     2103      -43     
==========================================
+ Hits         8904     9007     +103     
+ Misses       5879     5812      -67     
+ Partials      582      547      -35     

Impacted Files	Coverage Δ
packages/lu/src/parser/lubuild/builder.ts	12.80% <0.00%> (ø)
packages/lu/src/parser/lubuild/core.ts	16.29% <ø> (ø)
packages/lu/src/parser/qnabuild/builder.ts	22.72% <0.00%> (-0.71%)	⬇️
packages/qnamaker/src/commands/qnamaker/build.ts	76.84% <ø> (ø)
packages/lu/src/parser/cross-train/crossTrainer.js	92.30% <100.00%> (-2.41%)	⬇️
packages/lu/src/parser/qna/qnamaker/kbCollate.js	90.65% <100.00%> (+15.65%)	⬆️
packages/luis/src/commands/luis/build.ts	80.00% <100.00%> (ø)
...mathematics/confusion_matrix/AppConfusionMatrix.ts	6.66% <0.00%> (-22.32%)	⬇️
packages/luis/src/commands/luis/index.ts	66.66% <0.00%> (-16.67%)	⬇️
packages/qnamaker/src/commands/qnamaker/index.ts	66.66% <0.00%> (-16.67%)	⬇️
... and 125 more

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