plover_plugins crashes on run when run without arguments

[hlieberman]

Describe the bug

plover_plugins crashes when run either directly as a command or via the plover -s invocation when run without arguments.

Example:

❯ plover -s plover_plugins
Unexpected error: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/plover/scripts/main.py", line 115, in main
    code = entrypoint.load()()
  File "/usr/lib/python3/dist-packages/plover/plugins_manager/__main__.py", line 85, in main
    if args[0] == "list_plugins":
       ~~~~^^^
IndexError: list index out of range

Right now, we're indexing into argv without checking to see if there's anything in it. In general, this script should be fleshed out a bit more -- it should have a help, for example. For now, I'm just excluding its creation from the Debian package.