[v2] Add ability to override tsconfig

Type of Issue

[ ] Bug Report
[X] Feature Request

Description

I know that the idea of ng-packagr is to remove as much config as possible from the user but there are cases where being able to manage the tsconfig used by ng-packagr would be very useful, for example adding paths or alternate/additional typeroots. There could be an additional and optional flag to point to the desired tsconfig.

It can cause some difficult to troubleshoot issues when you need to use one config to test your library and another unseen one is used to build.

ng-packagr: v1.5.0
node: v8.6.0
@angular: v5.0.0
rxjs: 5.2.2
zone.js: latest

Thanks for the awesome work! I might be able to help with a PR if it is something @dherges thinks it is a valuable usecase

[villecoder]

@dherges How do you use the API? I don’t see a sample in the repository. What tool are you using to execute the sample code?

[DavidParks8]

This is very much related to my https://github.com/dherges/ng-packagr/issues/162#issuecomment-335665479. I would like to reiterate my proposal here and see if people agree. Essentially, I propose we add another config property to allow the specification of an override tsconfig:

{
  "tsconfig": "./../tsconfig.app.json"
}

This property would have to be clearly documented such that everyone is aware of what can, and can not be overridden in the tsconfig.json file.