PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Right now, the http module is used for any purpose inside core and at the same time imports other modules from the same scope (e.g. ImageSource).
The same module is both for internal and public usage.

What is the new behavior?

Separate http response content functionality and create the http-request-internal module for internal use.
This ensures we expose the same http api for users while having a minimal http client to use internally.

Separating those comes with benefits:

• Added missing types
• Duplicate functions cleanup
• Removed a wrong promise reject call from toFile that attempted to reject the fulfilled promise of the request itself