fix import filtered intents not work in luis:cross-train

[cosmicshuai]

closes: #1210
In this PR, for the test cases of using imported intents User report case will only includes the referenced intents in the generated files.

Here is the new result of Cross-train for that case above:

> LUIS application information
> !# @app.versionId = 0.1
> !# @app.culture = en-us
> !# @app.luis_schema_version = 3.2.0


> # Intent definitions

# InternalIntent
- Intent within application.lu


# ExternalIntentShouldBeIncluded
- Intent which should be included


> # Entity definitions


> # PREBUILT Entity definitions


> # Phrase list definitions


> # List entities

> # RegEx entities

[munozemilio]

Small comment on code optimization

[joshgummersall]

const, also does this need to be a deep clone?

[cosmicshuai]

The reason I used a deep clone here is due to I don't want to change the original content of luis and qna files.

[joshgummersall]

This isn't a deep clone actually, that was my question. See Typescript playground for explanataion.

[cosmicshuai]

I see. I just fixed this.