It could be added as an option to the windows installer, though rustup will eventually basically become the windows installer. For distros, I don't have any specific knowledge about their plans, but I consider this arrangement of the standard library source code to effectively be part of the broader Rust platform definition, and would strongly encourage them to support it in some way. sysroot/lib/rustlib/src is the canonical location to find the source to the standard library.
This reminds me that wouldn't it be great to have rust packaging guidelines.