We don't need to nest `cleanCargoSource` and `path` just to populate a
default value for `name`. As they both ultimately delegate to
`builtins.path`, the nesting can lead to IFD in situations which are
otherwise avoidable
* Turns out if a build is invoked with `--store` then the source
cleaning filter will observe paths rooted at the alternative store
which breaks out previous string handling (and results in incorrectly
ignoring files which should be included)
