Handle duplicated plugin install

[rootwarp]

• parent: CLIs for plugin #316

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Show error message if the user try to install the plugin that is already installed.

Ref. #317 (comment)

[xellos00]

@rootwarp
Back in our past discussion, we tried to use plugin_id due to its name duplication in different versions and then I thought it's a good idea to create a new primary flag which was plugin_id.

I've tried a new identifier which was plugin_id to prevent the installation of a duplicate plugin while I was developing PR #428 but decided to remain in current status using the plugin name that user has put as primary key. It's difficult for VATZ to extract plugin_id to recognize which plugin must be skipped if the user set enable/disable plugin status.

I think it's good to stick around with the current primary key name for the plugin table. I am going to close this issue unless you have any other comment or further discussions.
Please, let me know.

[xellos00]

@rootwarp
I close this issue, because seems you don't have any concern with it.