Uppercase letters in package names cause rextendr to generate incorrect `useDynLib` specification

If you call `rextendr::use_extendr` in a package with a capital letter, the generated Roxygen comment will contain the capital letters, e.g. ``` #' @useDynLib GTFSTest, .registration = TRUE ``` but the rust project will be all lower case. The package will then fail to work with errors about the `wrap__` functions not being found (though using `.Call` directly with quoted function names works, so the dylib is getting loaded but the functions aren't getting registered).