pgk_tar: package_dir = "./" attribute is not applied to pkg_mkdir

[ylecornec]

This is issue is similar to #610 but concerns pkg_mkdir instead of pkg_mklink.

How to reproduce

load("@rules_pkg//:pkg.bzl", "pkg_tar")
load("@rules_pkg//:mappings.bzl", "pkg_mkdirs", "pkg_files")

pkg_files(
    name = "myfile_pkg",
    srcs = ["myfile"],
)

pkg_mkdirs(
    name = "mydir",
    dirs = ["mydir"],
)

pkg_tar(
    name = "myarchive",
    srcs = ["myfile_pkg", "mydir"],
    package_dir = "."
)

invoking

bazel build myarchive.tar
tar tf bazel-bin/myarchive.tar

returns:

mydir/
./myfile

and not:

./mydir/
./myfile