backtrace: Shorten paths to crates/libcore about probe-run

[japaric]

For core/alloc/std libraries, a format stating the version of the compiler used in a format similar to rust-toolchain file specifications would be good

[stable-1.46.0]/lib/rustlib/src/foo.rs

(cross posting #65 (comment) here)

the contents file $(rustc --print sysroot)/share/doc/rust/html/version_info.html provided by the rust-docs component can be used to map the git hash in /rustc/$HASH paths to a Rust version.

But if we can get debuginfo remapping to work reliably (#65 (comment)) then the channel name is path of the local path, e.g. (..)/toolchains/stable-x86_64-unknown-linux-gnu. The version appears to be present in rustup manifests within that folder

$ rg '1\.52\.1'
manifest-rustc-x86_64-unknown-linux-gnu
22:file:lib/libLLVM-12-rust-1.52.1-stable.so
(..)