Remove subcommands from main help output

`license_finder help` was showing subcommands (dependencies add/remove).
This cluttered the help output and was confusing because it didn't
properly namespace the commands (it showed 'license_finder add' instead
of 'license_finder dependencies add').  This fix removes the subcommands
from the primary help.  You can still see help for the subcommands by
running `license_finder dependencies help`.