[plugin-main-file-location] [enhancement] Enriched the main plugin fi…

[vovafeldman]

…le finding when a plugin is programmatically activated with the following call stack: "PluginX -> ThemeX -> activate_plugin(PluginY)". Previously, the SDK picked up PluginX's main file as the main file of PluginY. This is relevant for companion theme plugins that can install plugins (that use Freemius) when importing starter sites.