Move plugin to nested folder for easy installation

Moved the plugin files to the directory structure they will have in the
python site-packages directory, being placed within the plugs directory
of the pubs application directory.

pyproject.toml

@@ -5,7 +5,7 @@ description = "A pdf annotation extraction plugin for pubs bibliography manager"
 authors = ["Marty Oehme <marty.oehme@gmail.com>"]
 license = "LGPL-3.0"
 readme = "README.md"
-packages = [{include = "extract"}]
+packages = [{include = "pubs"}]
 
 [tool.poetry.dependencies]
 python = "^3.10"