Remap relative bin-dir prefix for file!() and log macros

[tamasvajk]

Summary

Companion to #4016. While #4016 fixes --remap-path-prefix for absolute paths in debug info (backtraces, panic locations), this PR adds a remap for the relative bazel-out/<config>/bin/ prefix so that file!(), Location::caller(), and tracing/log macros also produce clean workspace-relative paths when sources are symlinked by transform_sources().

Problem

When a target mixes generated and non-generated sources, transform_sources() symlinks all source files into bazel-out/<config>/bin/.... The existing --remap-path-prefix=${pwd}=. only affects absolute paths embedded in debug info. However, file!(), Location::caller(), and log macros embed the literal relative path that rustc received on the command line. These are not matched by the ${pwd} remap, so they produce paths like:

bazel-out/k8-fastbuild/bin/my/pkg/lib.rs

instead of:

my/pkg/lib.rs

Fix

When crate_info.root.is_source is False (symlinked sources), add a second remap that strips the relative bin-dir prefix:

--remap-path-prefix=bazel-out/<config>/bin/=

This complements the existing absolute-path remaps and ensures compile-time paths are also clean.

Tests

• Analysis tests: Verify the relative bin-dir remap flag is present for generated-source targets and absent for plain source targets.
• Runtime test: A library with generated sources exposes file!() and a test asserts the returned path does not contain bazel-out.