Default naming convention for WIT package is broken on Windows (includes a semicolon)

Trying to build WIT packages on Windows requires setting the output file name manually via `-o/--output`, as otherwise `wkg` creates a blank file named after the namespace. This is due to Windows' file name convention, which doesn't allow certain characters, including semicolons.

Could a different default naming convention be used instead for build files? Like treating the namespace as a directory and storing all of the namespaced packages inside it? Or change the semicolon to a character that is cross-platform friendly like --, _ or +?