False positive when `-` in crate name

[ralpha]

We use iso639-1 in our tool. But this is flagged as unused. This is most likely because of the - in the crate name. The actual use is iso639_1::something()

So the - is replaced with a _ in the code. If this is a common problem maybe there can a special case for all the crates that have a - in there name.

For now I just added it to the ignore list:

[package.metadata.cargo-udeps.ignore]
normal = ["iso639-1"]