Submit should warn users that try to check in a directory

[mhelmetag]

Brought up in exercism.io#2802.

It could be a good idea to give users a heads up when they try to submit a directory.

This could help funnel new users towards the correct usage of the cli (along with the existing control flow in submit already).

[kytrinyx]

Agreed, this would be a very nice fix. It also shouldn't be difficult to implement.

[alebaffa]

I can take care of this one :) it would be my first PR here on exercism.

I took a look and it seems pretty simple to implement.
I think I could add a new check in cli/cmd/submit.go along with these existing ones:

if isTest(filename) ...
if isREADME(filename)...

I see two solutions, but I would like to hear your opinion:

1. to create a new function (which will be stored in /cmd/cmd.go)

if isDirectory(filename)

2. To use the function

func IsDir(path string) bool

which is already implemented and stored in paths/paths.go

The 1) allows to stay coherent with the previous coding style (isTest, isREADME), the 2) allows to reuse an existing function.

What do you suggest?

[kytrinyx]

I would suggest using the IsDir function, since it already does what you need.

[mhelmetag]

@alebaffa , thanks for this :D