inconsistent behavior of --link flag during plugin add.

Hi,

When I add a plugin from local using --link as the documentation mentions, I notice that the plugins folder in the application base folder is a symlink. That is gud :) , but.....

At the `platform_www/plugins` 2 unexpected behaviour can happen:

- if platform installed first & then plugin added: a copy of plugin used and not a symlink
- if plugin installed first & then platform: any plugin added by --link is totally forgotten about & missing

cordova --version : 7.0.1
ios@~4.5.5
browser@~4.1.0

Is this is expected behaviour or an actual bug?

Thanks!

Reproduce both installation orders with Cordova CLI 7.0.1, ios ~4.5.5, and browser ~4.1.0, adding a local plugin with --link. Inspect the application-base plugins folder and platform_www/plugins, then trace the plugin-add and platform-install entry points and existing tests. Done means the intended linked-plugin behavior is defined and covered for both orders.

Written by the indexing model from the issue text.