CustomizedImportResolver in LU build/crossTrain

[zhixzhan]

Composer use are use those modules do publishing:

const crossTrainer = require('@bfcomposer/bf-lu/lib/parser/cross-train/crossTrainer.js');
const luBuild = require('@bfcomposer/bf-lu/lib/parser/lubuild/builder.js');
const LuisBuilder = require('@bfcomposer/bf-lu/lib/parser/luis/luisBuilder');

3 methods need support customized import resolver:

• loadContents in lubuild.Builder and qnabuild.Builder
• crossTrain in crossTrainer