Auto-detecting executable bit in `pkg_tar` can't be used in conjunction with `pkg_files`

The `build_tar.py` has logic to auto-detect the executable bit of the source files and also apply that in the final tar file:
```
# If mode is unspecified, derive the mode from the file's mode.
if mode is None:
f_mode = 0o755 if os.access(content_path, os.X_OK) else 0o644
else:
f_mode = mode
```
https://github.com/bazelbuild/rules_pkg/blob/main/pkg/private/tar/build_tar.py#L283

This logic cannot be reached however when using `pkg_tar` in conjunction with `pkg_files`, as the `_pkg_files_impl` doesn't allow `None` to pass through, it overwrites it with a `0644` default.

```
# The least surprising default mode is that of a normal file (0644)
out_attributes.setdefault("mode", "0644")
```
https://github.com/bazelbuild/rules_pkg/blob/main/pkg/mappings.bzl#L256

Is that default there on purpose? There is another default for `mode` in `pkg_tar` of `0555`. So the default in `pkg_files` make the default of `pkg_tar` not usable.

The initial problem could be solved by simply removing that default in `_pkg_files_impl`, then this code would be using the auto-detection of the executable bit:
```
pkg_files(
name ="files",
srcs = [src],
)

pkg_tar(
name = package,
srcs = ["files"],
mode = None,
)
```

For the case where neither `pkg_files` nor `pkg_tar` specify a mode, the files are created with the default mode of tar (555).

Would that make sense? I didn't look at the other packaging rules yet how they would be impacted.