Panic on duplicate command names

[michal-kralik]

Closes #300

Adds supports for validating if command names are not duplicated within a single node's children.
Panics if they are duplicated.

This is a breaking change in case a project has duplicated command names.
Positional arguments are not affected and can have duplicated names.

[alecthomas]

LGTM just a minor linter issue - feel free to disable gocyclo in .golangci.yml

[michal-kralik]

@alecthomas I resolved the gocyclo error. Once the workflow runs, this shall be ready for merging.

On a side note - I wanted to add the same check for aliases vs. names but there's a test which fails afterwards.
I'm not sure if it's expected behavior. In this case the Cmd does not run, but OtherCmd is invoked.
My understanding is aliases trigger the same command (Cmd in this case) just with a different name.

In this example it would not be clear which command to run:

var cli struct {
  CmdA Command `cmd aliases:"my-cmd"`
  CmdB Command `cmd aliases:"my-cmd"`
}

I'm happy to submit a PR for aliases check as well if you agree.

[alecthomas]

Mmm good point, duplicate aliases shouldn't be allowed either. A PR would be great!