The `mode` attribute overwrites the file mode for _all_ files. This is usually
not desirable (e.g., some files are executable, some aren't). Additionally, a
default value of 0555 applies executable permissions to all files (not ideal)
and takes away user write permission (worse).

The default PAX format has issues with long file paths. If the complete path to
a file, including the package_dir prefix, exceeds 100 characters, Python will
incorrectly truncate it so the resulting path within the tar file does not have
the package_dir prefix. For example:
  File: path/to/file.txt
  Package dir: my_dir
  Expected path: my_dir/path/to/file.txt
  Resulting path: path/to/file.txt (if expected path > 100 chars)

…ion.

The `rules_pkg` mainline has the behaviour where it doesn't allow prefix duplication where a subfolder name mimics the top level directory name. Since sometimes this directory structure is desired, a bazel level option was added to control the behaviour.

## Features
- Added the `allow_prefix_duplication` flag to tar creation to allow calls to pkg_tar in bazel to control the desired directory structure. This flag defaults to false to mimic the mainline behaviour.

Merge Pull Request #2.