Fix behavior of install-n command

Commit de9643a (first shipped in dnf 4.2.8) changed opts.command
from a list to a simple string, but code in InstallCommand still handled
it as if it was a list.

This broke silently because Python will iterate over a string character
by character, so it never produced a syntax error or an exception that
would be noticed.

But it broke behavior of the `install-n` form, since it would no longer
detect that command to restrict search of packages.

Before this commit, this command would (incorrectly) succeed:
  $ dnf install-n joe-4.6

After this commit:
  $ dnf install-n joe-4.6
  No match for argument: joe-4.6
    * Maybe you meant: joe
  Error: Unable to find a match: joe-4.6

Closes: #1611
Approved by: m-blaha

dnf/cli/commands/install.py

@@ -120,10 +120,10 @@ def run(self):
                                                            packages=err_pkgs)
 
     def _get_nevra_forms_from_command(self):
-        return [self.nevra_forms[command]
-                for command in self.opts.command
-                if command in list(self.nevra_forms.keys())
-                ]
+        if self.opts.command in self.nevra_forms:
+            return [self.nevra_forms[self.opts.command]]
+        else:
+            return []
 
     def _log_not_valid_rpm_file_paths(self, grp_specs):
         group_names = map(lambda g: '@' + g, grp_specs)