feat(typescript_indexer): implement ref inlining for imported symbols

[nbeloglazov]

The goal of this PR is to connect usages of imported symbol to the its declaration from original file. Today indexer produces the following graph:

// foo.ts
//- @ANSWER defines/binding AnswerOrig
export const ANSWER = 42;


// bar.ts
//- @ANSWER ref/imports AnswerOrig
//- @ANSWER defines/binding AnswerLocal
import {ANSWER} from './foo';

//- @ANSWER ref AnswerLocal
console.log(ANSWER);

After this PR the graph will look like this:

// foo.ts
//- @ANSWER defines/binding AnswerOrig
export const ANSWER = 42;


// bar.ts
//- @ANSWER ref/imports AnswerOrig
import {ANSWER} from './foo';

//- @ANSWER ref AnswerOrig
console.log(ANSWER);

Note that AnswerLocal is gone.

The change is guarded by enableImportsEdgeReassignment flag. This will allow us to easier roll it out at Google.

This is related to #4021.

[nbeloglazov]

@jaysachs bump

[jaysachs]

Looks like some trailing whitespace in comments in indexer.ts