The way packages are stored internally has changed

Previous versions of dub stored packages in the following format:
`$CACHE_PATH/$PACKAGE_NAME-$PACKAGE_VERSION/$PACKAGE_NAME/`
Starting from this version, the format will be:
`$CACHE_PATH/$PACKAGE_NAME/$PACKAGE_VERSION/$PACKAGE_NAME`.

Introducing a new level will help users quickly list what packages
they actually have installed, and reduce visibility of packages that
might update frequently. It will render various commands (e.g. `du`)
more useful, pave the way for a package GC function, and make manual
browsing easier.

More importantly, it will allow future version of dub to infer the
version from the path to the package, removing the need to read (or edit)
the recipe file on every dub invocation.