Description of problem:

The doc/examples/install_plugin.py example does not work out of box.

Version-Release number of selected component (if applicable):

Testing on Fedora rawhide with dnf-4.0.10-1.fc30.noarch.

How reproducible:

Deterministic.

Steps to Reproduce:
1. Copy doc/examples/install_plugin.py from dnf master to /usr/lib/python3.7/site-packages/dnf-plugins/foo.py
2. Try to test with dnf foo zsh

Actual results:

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 95, in _main
    cli.configure(list(map(ucd, args)), option_parser())
  File "/usr/lib/python3.7/site-packages/dnf/cli/cli.py", line 940, in configure
    self.optparser.print_help(self.command)
  File "/usr/lib/python3.7/site-packages/dnf/cli/option_parser.py", line 407, in print_help
    cp = self._command_parser(command)
  File "/usr/lib/python3.7/site-packages/dnf/cli/option_parser.py", line 341, in _command_parser
    command.set_argparser(self)
  File "/usr/lib/python3.7/site-packages/dnf-plugins/foo.py", line 29, in set_argparser
    parser.add_argument('package', nargs='+', metavar=_('PACKAGE'),
NameError: name '_' is not defined

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 95, in _main
    cli.configure(list(map(ucd, args)), option_parser())
  File "/usr/lib/python3.7/site-packages/dnf/cli/cli.py", line 940, in configure
    self.optparser.print_help(self.command)
  File "/usr/lib/python3.7/site-packages/dnf/cli/option_parser.py", line 407, in print_help
    cp = self._command_parser(command)
  File "/usr/lib/python3.7/site-packages/dnf/cli/option_parser.py", line 341, in _command_parser
    command.set_argparser(self)
  File "/usr/lib/python3.7/site-packages/dnf-plugins/foo.py", line 30, in set_argparser
    action=OptionParser.ParseSpecGroupFileCallback,
NameError: name 'OptionParser' is not defined

Last metadata expiration check: 1:46:16 ago on Fri 18 Jan 2019 05:56:29 AM EST.
Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 99, in _main
    return cli_run(cli, base)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 115, in cli_run
    cli.run()
  File "/usr/lib/python3.7/site-packages/dnf/cli/cli.py", line 1107, in run
    return self.command.run()
  File "/usr/lib/python3.7/site-packages/dnf-plugins/foo.py", line 44, in run
    self.base.package_install(self.base.add_remote_rpms(self.opts.package, strict=False))
  File "/usr/lib/python3.7/site-packages/dnf/base.py", line 1891, in package_install
    q = self.sack.query()._nevra(pkg.name, pkg.evr, pkg.arch)
AttributeError: 'list' object has no attribute 'name'

Expected results:

No errors, the example does something useful.

Additional info:

Fix for the first two tracebacks is in https://github.com/rpm-software-management/dnf/pull/1304.

But I have no idea how the self.base.package_install(self.base.add_remote_rpms(self.opts.package, strict=False)) line should work or what it's supposed to do.