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

This is issue is similar to https://github.com/bazelbuild/rules_pkg/issues/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
```