Support .plugged within addons folder

[Tattomoosa]

Fixed the issue in https://github.com/imjp94/gd-plug/pull/33#issuecomment-2309466681 , in this implementation whether or not to include/exclude files is only determined from the plugin root. This seems like the sanest behavior to me. This fixes #32

For more information on what the issue with that implementation, default "addons" include was matching on "res://addons/gd-plug/.plugged" and attempting to copy all plugin files, then aborted when file overwrite was attempted.

This might need another change to handle the case of copying files from the project root, but I'm not aware of any case where gd-plug should ever be doing that, so not sure how to go about testing that case.