Store plugin path and avoid dupes

[trufae]

trafficstars

there should be a warning when trying to open a secondary plugin with the same name+type. also Lcj should display the absolute path where each plugin is loaded from

[trufae]

or maybe checksum the plugin file which can be probably a better approach, because r2 could be reporting better error messages when finding duplicated plugin installations (in exchange for slower loading times :__)

[trufae]

let's move this to the next abi breaking release, not really a big issue if users keep their plugin folders clean. and we may probably want to find a better balance between startup performance and so on