External files are not livesynced

[rosen-vladimirov]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 6.4.0
• Cross-platform modules: Not applicable
• Android Runtime: Not applicable
• iOS Runtime: Not applicable
• Plugin(s): Not applicable

Describe the bug
In case you have defined externals in your webpack.config.js, changing the code in those externals is not livesynced, so the changes are not visible on device. You have to restart the command to apply the changes, which makes the process slow.

To Reproduce
Use the project here - https://github.com/rosen-vladimirov/appWebpackExternalsIssue - check the README for description.

Expected behavior
Changes in externals must be visible and should not require restart of CLI command.

Sample project
https://github.com/rosen-vladimirov/appWebpackExternalsIssue

Additional context
Fix may result in regression in #5105

[dtopuzov]

Fix available in nativescript@rc, will be shipped with [email protected]