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`

devtools/parse_pcap.py

@@ -8,7 +8,7 @@ import click
 import dpkt
 from dpkt.ethernet import ETH_TYPE_IP, Ethernet
 
-from kasa.cli import echo
+from kasa.cli.main import echo
 from kasa.xortransport import XorEncryption