rust-src component is not self-contained anymore

[oli-obk]

Since #73441 I'm getting a failure while building libstd.

error: couldn't read /home/oliver/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/../backtrace/src/lib.rs: No such file or directory (os error 2)
   --> /home/oliver/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/lib.rs:519:1
    |
519 | mod backtrace_rs;
    | ^^^^^^^^^^^^^^^^^

error: aborting due to previous error

Should we copy the backtrace folder into the rust-src rustup component or is there a better solution?

cc @alexcrichton

[RalfJung]

Yeah I think we need to do that, and we should do it ASAP. We can always find a better version later.