New remap rules for git/registry dependencies:
* Git dependencies: remove ~/.cargo/git/checkouts prefix.
* Registry dependencies: remove ~/.cargo/registry/src prefix.
This make the remap rules fixed to a finite number,
minimizing the burden for users to configure debuggers.
In https://github.com/rust-lang/rust/blob/87e1447aa/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs#L856
when the remap result of `work_dir` is an empty string,
LLVM won't generate some symbols for root debuginfo node.
For example, `N_SO` and `N_OSO` on macOS,
or `DW_AT_comp_dir` on Linux when debuginfo is splitted.
Precisely, it is observed that when the `DIFile` of compile unit was
provied with an empty compilation `Directory` string,
LLVM would not emit those symbols for the root DI node.
This behavior is not desired,
resulting in corrupted debuginfo and degrading debugging experience.
This is might not be a bug of `--remap-path-prefix` in rustc,
since `-fdebug-prefix-map` in clang 16 could have the same result
(`DW_AT_comp_dir` is gone when `work_dir` is remapped to an empty string).
However, in gcc 12 `fdebug-prefix-map` will return an absolute work_dir
when an empty string occurs.
To not bother whether this needs to be fixed in rustc or not,
let's fix it by always appending an explicit `.`
when `--remap-path-prefix` remaps to relative workspace root
a.k.a. where rustc is invoking.
For more on gcc/clang remap options, see
https://reproducible-builds.org/docs/build-path/
