cargo: Line numbers in backtrace still do not appear on macOS for Cargo projects

Repro steps:

1. Create a cargo project.

   cargo new --bin a && cd a && echo 'fn main() { panic!(); }' > src/main.rs 
   

2. Run it. Note that the line numbers are missing (unexpected).

   RUST_BACKTRACE=1 cargo run
   

   Output

   thread 'main' panicked at 'explicit panic', src/main.rs:1:12
   stack backtrace:
      0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
      1: std::sys_common::backtrace::_print
      2: std::panicking::default_hook::{{closure}}
      3: std::panicking::default_hook
      4: std::panicking::rust_panic_with_hook
      5: std::panicking::begin_panic
      6: a::main
      7: __rust_maybe_catch_panic
      8: std::rt::lang_start
      9: main
   

3. Run the actual hard-linked program. The line numbers now appear (expected).

   RUST_BACKTRACE=1 target/debug/deps/a-*
   

   Output

   thread 'main' panicked at 'explicit panic', src/main.rs:1:12
   stack backtrace:
      0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
                at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
      1: std::sys_common::backtrace::_print
                at src/libstd/sys_common/backtrace.rs:71
      2: std::panicking::default_hook::{{closure}}
                at src/libstd/sys_common/backtrace.rs:60
                at src/libstd/panicking.rs:381
      3: std::panicking::default_hook
                at src/libstd/panicking.rs:397
      4: std::panicking::begin_panic
                at src/libstd/panicking.rs:577
      5: std::panicking::begin_panic
                at /Users/travis/build/rust-lang/rust/src/libstd/panicking.rs:538
      6: a::main
                at src/main.rs:1
      7: panic_unwind::dwarf::eh::read_encoded_pointer
                at src/libpanic_unwind/lib.rs:99
      8: rust_panic
                at src/libstd/panicking.rs:459
                at src/libstd/panic.rs:361
                at src/libstd/rt.rs:59
      9: a::main
   

The problem is that in rust-lang/rust#44251 we did not search for the deps/ folder. The fix is either,

1. Symlink target/debug/deps/a-«hash».dSYM to target/debug/a-«hash».dSYM (actual name irrelevant), don’t touch libstd. This may also fix #4056; or
2. Amend rust-lang/rust#44251 to look for deps/ as well, don’t touch cargo.

cc @JohnColanduoni

$ cargo -vV
cargo 0.23.0-nightly (33250c48b 2017-09-09)
release: 0.23.0
commit-hash: 33250c48b4763b01478d780e76206484a1d5b207
commit-date: 2017-09-09

$ rustc -vV
rustc 1.22.0-nightly (539f2083d 2017-09-13)
binary: rustc
commit-hash: 539f2083de809b5c8304fe7426655cfeb0e66d5e
commit-date: 2017-09-13
host: x86_64-apple-darwin
release: 1.22.0-nightly
LLVM version: 4.0