Auto merge of #1620 - Eljay:empty-package-name, r=alexcrichton

Some simple validation for package/target names. Currently failure for these cases happens later in rustc (e.g. "crate name must not be empty") or at the os level (e.g. "Is a directory" error when package name is empty on linux). Better to handle this earlier so it's consistent across platforms.

Still could do with a lot of validation for invalid characters etc. though.