Warn user if there's a file cross.toml with wrong capitalization

[denisfalqueto]

I've lost some good days struggling with errors because I've created a file named cross.toml and it wouldn't affect the build, because the name was not capitalized correctly. So, I think it should be cool if cross could warn the user if there's a file with a capitalization problem present on the project root.

[otavio]

@maintainers, this seems very simple and useful.

[Emilgardis]

thank you!

bors r+

[bors]

Build succeeded:

• conclusion