`write_source_files` misclassifies missing or dangling inputs as directories

`write_source_files` determines whether an input is a file using this binary check:

```bash
if [[ -f "$in" ]]; then
# copy file
else
# copy directory
fi
```

The else branch assumes that anything not recognized as a regular file must be a directory. However, `[[ -f ]]` also returns false when the input is missing or is a dangling symlink.

This produces a misleading failure. The updater reports that it is copying a directory and may eventually fail with an unrelated error such as:

```text
mkdir: cannot create directory '': File exists
```

The actual problem is that the input is unavailable.

The script should distinguish all three states explicitly:

```bash
if [[ -f "$in" ]]; then
# copy file
elif [[ -d "$in" ]]; then
# copy directory
else
echo "Input is unavailable or is a dangling symlink: $in" >&2
exit 1
fi
```

This would not materialize remotely stored Bazel outputs or otherwise fix their absence. It would provide an accurate, actionable diagnostic instead of treating the missing input as a directory.

Locate the write_source_files entry point and inspect its input-state checks. Verify behavior for a regular file, directory, missing input, and dangling symlink; done means unavailable inputs produce the specified actionable diagnostic on stderr and exit without being treated as directories.

Written by the indexing model from the issue text.