allow ::current_plugin_loadpath to be set for manually loaded .tcl scripts

[porres]

currently this "::current_plugin_loadpath" variable is only set for "actual plugins" which are tcl plugins loaded when they are named in the given format, like terminating with "plugin.tcl".

But I am now working with objects that load particular plugin scripts via "load_plugin_script" and when I do this the "::current_plugin_loadpath" variable is not set and I'd like to be able to access the directory where the .tcl file is.

I don't think it hurts to be able to also set "::current_plugin_loadpath" in this case, what do you think?