Externalize collectExternalModuleReferences

[s-panferov]

Refs #9017

Right now there is no way to extract external dependencies from SourceFile. This is useful for tools like awesome-typescript-loader to track files dependencies and trigger rebuils on changed files.

I propose to make collectExternalModuleReferences function from program.ts to became public for external users or add overload to preProcessFile to take existing SourceFile.

[RyanCavanaugh]

You can already simply walk the source tree (see here https://github.com/Microsoft/types-publisher/blob/master/src/lib/definition-parser.ts#L240 ) to collect the import statements. Is there extra information only the TS compiler itself could provide?