fix: send hash per platform when more than one webpacks are stated

[Fatme]

Hmr hash is not correct when more than one webpacks are started. So send hash per platform in order to ensure {N} CLI will receive the correct hash.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-dev-webpack/blob/master/CONTRIBUTING.md#testing-locally-by-running-e2e-tests
• Tests for the changes are included.

What is the current behavior?

{N} CLI does not receive correct file's hash when 2 webpacks are started.

What is the new behavior?

{N} CLI receives the correct file's hash when 2 webpacks are started.