feat: stop using the proxy tns-core-modules package when the @nativescript/core is available

[DimitarTachev]

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.
• You have signed the [CLA].
• 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?

It's not possible to use the plugin without the tns-core-modules proxy package because we inject tns-core-modules require statements in the app.

What is the new behavior?

The plugin injects the require statements based on the @nativescript/core package availability.

P.S. The PR is based on the @DickSmith's proposal in this PR with two additional improvements.

1. The code is meant to be backward compatible (it should work with core modules before 6.2.0.)
2. The "short imports" syntax is also supported in the "scoped packages only" flow.

Related to: #1089

[DimitarTachev]

test cli-smoke cli-templates cli-plugin cli-preview cli-tests cli-vue cli-build cli-debug cli-device cli-run webpack-apps

[rosen-vladimirov]

test cli-smoke cli-templates cli-plugin cli-preview cli-tests cli-vue cli-build cli-debug cli-device cli-run webpack-apps

[Fatme]

test cli-smoke cli-templates cli-plugin cli-preview cli-tests cli-vue cli-debug cli-device cli-run

[Fatme]

test cli-templates cli-preview cli-test cli-vue cli-debug cli-device cli-run

[Fatme]

test cli-templates cli-preview cli-test cli-vue cli-debug cli-device cli-run

[Fatme]

test cli-smoke cli-templates cli-plugin cli-preview cli-tests cli-vue cli-build cli-debug cli-device cli-run webpack-apps