Error if no packs in pack folder on draft create

[thatarchguy]

For new users, forgetting to run draft init is not as obvious as it should be. This PR throws an error if the packs directory is empty when running draft create, which is normally caused by not running draft init first.

Fixes: https://github.com/Azure/draft/issues/925

Before:

$ draft create
--> Draft detected Go (100.000000%)
--> Could not find a pack for Go. Trying to find the next likely language match...
Error: no languages were detected

After:

$ draft create
Error: No packs found in /home/klaw/.draft/packs, did you `draft init`?