A dependency on `path = "."` should have a good error message

[Eh2406]

Blocked on #2911

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In a package foo a [dev-dependencies] foo = { path = "." } should be an error.
As there is no use case for it, and it does not do what most people expect.
There are not many people that will try this, and there are few configurations that don't error already. So I don't think we need to get too fancy with the comparison. A check for a path "." would be a big step foreword.

[Eh2406]

cc #9245 and #3921 (comment)

[Nemo157]

Will probably fix #9454 too (by making it an error).

[yerke]

@rustbot claim

[yerke]

Does anyone have an example of a good error message we want to show in this case?

[Eh2406]

I don't have a good suggestion, but how does this sound:
"a project can not depend on itself as a path dependency"

[yerke]

@Eh2406 Great, thanks! It's better than "Something went wrong" I planned to use as a placeholder :)