When Cargo.toml is a symlink, cargo add was overwriting it with a regular
file. This change follows the symlink and writes to the target file instead,
preserving the symlink structure.

Fixes rust-lang#15241

When Cargo.toml is a symlink, cargo add was overwriting it with a regular
file. This change follows the symlink and writes to the target file instead,
preserving the symlink structure.

Fixes rust-lang#15241

When Cargo.toml is a symlink, cargo add was overwriting it with a regular
file. This change follows the symlink and writes to the target file instead,
preserving the symlink structure.

Fixes rust-lang#15241

When Cargo.toml is a symlink, cargo add was overwriting it with a regular
file. This change follows the symlink and writes to the target file instead,
preserving the symlink structure.

Fixes rust-lang#15241

When cargo add is used on a project where Cargo.toml is a symlink,
it now follows the symlink and writes to the target file instead of
replacing the symlink with a regular file.

This is achieved by updating write_atomic() to detect symlinks and
resolve them before writing.

Fixes rust-lang#15241

- Preserve symlinks when writing files atomically in write_atomic()
- Update test to verify correct symlink preservation behavior
- Apply rustfmt formatting

This fixes the issue where cargo add would replace symlinked Cargo.toml
files with regular files, breaking the symlink to the original target.

Fixes rust-lang#15241

### What does this PR try to resolve?

This PR fixes a bug where `cargo add` breaks symlinks to Cargo.toml
files. Currently, when Cargo.toml is a symlink and `cargo add` is used
to add a dependency, the symlink is replaced with a regular file,
breaking the link to the original target file.

This issue was reported in #15241 where a user who relies on symlinked
Cargo.toml files found that `cargo add` breaks their workflow.

Fixes #15241

### How should we test and review this PR?

I've modified `LocalManifest::write()` to check if the path is a
symlink, and if so, follow it to get the actual target path. This
ensures we write to the actual file rather than replacing the symlink.

I've also added a test in `tests/testsuite/cargo_add/symlink.rs` that:
1. Creates a symlinked Cargo.toml file
2. Runs `cargo add` to add a dependency
3. Verifies the symlink is preserved and the dependency is added to the
target file

I've manually tested this fix and confirmed it works correctly.