Structure cli into a package (#1038)

PR with just the initial structural changes for the cli to be a package.
Subsequent PR will break out `main.py` into modules. Doing it in two
stages ensure that the commit history will be continuous for `cli.py` >
`cli/main.py`

kasa/cli/__main__.py

@@ -0,0 +1,5 @@
+"""Main module."""
+
+from kasa.cli.main import cli
+
+cli()