All commands now support `package[@<version-spec>]`, `--version` is deprecated

`dub` commands had some disparity in their version handling.
Most commands accepting a package name (e.g. `dub fetch` or `dub run`) supported
either `package[@version-spec]`, or supported only `package` and accepted an
option `--version=<version-spec>`.
The latter form has now been deprecated and all commands now accept `package[@<version-spec>]`.
Note that the absence of `@<package-spec>` still means "latest version" (except for `remove`).