pkg_tar.strip_prefix does not work for srcs from other packages.

```
# foo/BUILD
exports_files(glob(["testdata/**"]))
```

```
# my/BUILD
pkg_tar(
name = 'sample',
srcs = ['//a:testdata/foo'],
strip_prefix = "a/testdata",
)
```
You end up with a/testdata/foo in the archive.

The solution may be to just document this as a deficiency and tell users to use pkg_files.