fix(trim-paths): explicit remap to current dir .

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/

src/cargo/core/compiler/mod.rs

@@ -1230,7 +1230,7 @@ fn trim_paths_args(
         // * path dependencies outside workspace root directory
         if is_local && pkg_root.strip_prefix(ws_root).is_ok() {
             remap.push(ws_root);
-            remap.push("="); // empty to remap to relative paths.
+            remap.push("=."); // remap to relative rustc work dir explicitly
         } else {
             remap.push(pkg_root);
             remap.push("=");