Updating package.json platform versions, doesn't work.

Environment

• CLI: 7.x

Describe the bug
Basically because we are using NPM now to manage the dependancies; it will store a copy of the runtime in @nativescript/ios or @nativescript/android if you bump the version of the package.json file; it will bump those copies; but the copy that we create in the "platforms/ios" or "platforms/android" remains whatever was their before.

Expected behavior
That I would have the version in the package.json file.

Additional context
We might need a post install script to clean the platforms/ios or platforms/android folder on install of the runtimes, so that way the next tns command will see the platforms folder empty and re-install the platforms from the npm version that was installed.

Possible corner case is if the CLI installs the platforms version before doing a npm i; we need to verify this isn't the case.

Start by tracing how package.json updates the runtime copies in @nativescript/ios and @nativescript/android, then compare that with the existing platforms/ios and platforms/android handling. Verify whether the CLI installs platforms before npm i and assess the suggested post-install cleanup. Done means platform folders use the versions installed from package.json.