Function platform_plugin_directories 

pub fn platform_plugin_directories() -> Vec<PathBuf>

Returns configured plugin directories.

Uses conventional platform directories unless PLUGIN_DIRS_ENV_VAR is set, in which case the env path list replaces the conventional directories.

PLUGIN_DIRS_ENV_VAR uses operating-system path-list semantics via std::env::split_paths. An empty PLUGIN_DIRS_ENV_VAR value disables plugin directory scanning.