refactor: renamed query-builder/RelationIdLoader.ts to QueryStrategyRelationIdLoader.ts

[RohanTalip]

Description of change

Prior to this pull request, there were 2 classes named RelationIdLoader, in:

1. src/query-builder/RelationIdLoader.ts
2. src/query-builder/relation-id/RelationIdLoader.ts

This caused me some confusion recently.

The first one was referenced in src/data-source/DataSource.ts but otherwise not really used, so the usage has been removed in this pull request. It was also referenced in src/query-builder/SelectQueryBuilder.ts but renamed to QueryStrategyRelationIdLoader when importing:

typeorm/src/query-builder/SelectQueryBuilder.ts

Line 11 in e7649d2

import { RelationIdLoader as QueryStrategyRelationIdLoader } from "./RelationIdLoader"

Hence it made sense to me to rename the first RelationIdLoader to QueryStrategyRelationIdLoader, to match what it was already being renamed to anyway.

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run format to apply prettier formatting
• npm run test passes with this change -- N/A - see circleci tests
• This pull request links relevant issues as Fixes #0000 -- N/A
• There are new or updated unit tests validating the change -- N/A
• Documentation has been updated to reflect this change -- N/A
• The new commits follow conventions explained in CONTRIBUTING.md