Linking to a directory causes an error with cmake

```toml
[target.mylib]
type = "static"
sources = ["mylib/lib.cpp"]

[target.foo]
type = "executable"
sources = ["main.cpp"]
link-libraries = ["mylib"]
```

If `$(pwd)/mylib` is a directory cmkr generates `${CMAKE_CURRENT_SOURCE_DIR}/mylib` and CMake (rightfully) errors out.