Use pkg_tar to flatten tar files

I tried pkg_tar to flatten the tar files:

I created 2 test tar files:

```
gfrankliu@gfrankliu /tmp/test $ tar tvf file1.tar
lrwxrwxrwx gfrankliu/primarygroup 0 2024-06-21 17:30 bin -> ./usr/bin
drwxr-xr-x gfrankliu/primarygroup 0 2024-06-21 17:30 usr/
drwxr-xr-x gfrankliu/primarygroup 0 2024-06-21 17:30 usr/bin/
gfrankliu@gfrankliu /tmp/test $ tar tvf file2.tar
-rw-r--r-- gfrankliu/primarygroup 0 2024-06-21 17:11 ./bin/test
```

If I manually untar file1 and then file2, I get correct result:

```
gfrankliu@gfrankliu /tmp/test $ tar xf file1.tar
gfrankliu@gfrankliu /tmp/test $ tar xf file2.tar
gfrankliu@gfrankliu /tmp/test $ ls -l
total 28
lrwxrwxrwx 1 gfrankliu primarygroup 9 Jun 21 17:30 bin -> ./usr/bin
-rw-r--r-- 1 gfrankliu primarygroup 10240 Jun 21 17:30 file1.tar
-rw-r--r-- 1 gfrankliu primarygroup 10240 Jun 21 17:31 file2.tar
drwxr-xr-x 3 gfrankliu primarygroup 4096 Jun 21 17:30 usr
gfrankliu@gfrankliu /tmp/test $ ls -l bin/
total 0
-rw-r--r-- 1 gfrankliu primarygroup 0 Jun 21 17:11 test
gfrankliu@gfrankliu /tmp/test $
```

If I use pkg_tar:
```
load("@rules_pkg//:pkg.bzl", "pkg_tar")

pkg_tar(
name = "flatten",
deps = [
"file1.tar",
"file2.tar",
],
)
```

The resulting tar tvf flatten.tar looks like this:

```
lrwxrwxrwx 434078/89939 0 2024-06-21 17:30 bin -> ./usr/bin
drwxr-xr-x 434078/89939 0 2024-06-21 17:30 usr/
drwxr-xr-x 434078/89939 0 2024-06-21 17:30 usr/bin/
drwxr-xr-x 434078/89939 0 2024-06-21 17:11 ./bin/
-rw-r--r-- 434078/89939 0 2024-06-21 17:11 ./bin/test
```

If I extract the flatten.tar (tar -xvf ...), I get
```
gfrankliu@gfrankliu /tmp/flatten $ ls -lR
.:
total 8
drwxr-xr-x 2 gfrankliu primarygroup 4096 Jun 21 17:11 bin
drwxr-xr-x 3 gfrankliu primarygroup 4096 Jun 21 17:30 usr

./bin:
total 0
-rw-r--r-- 1 gfrankliu primarygroup 0 Jun 21 17:11 test

./usr:
total 4
drwxr-xr-x 2 gfrankliu primarygroup 4096 Jun 21 17:30 bin

./usr/bin:
total 0
```

As you can see, the symlink /bin got deleted, so pkg_tar doesn't seem to create the correct flatten tar file. The exploded directories from if I manually untar file1.tar, file2.tar, vs if I untar the flatten.tar look different.