tarsum: name collision fix

If a tar were constructed with duplicate file names, then depending on
the order, it could result in same tarsum.

This does not break the existing checksums of correctly produced layers.

Signed-off-by: Vincent Batts <vbatts@redhat.com>
This is still needed:
```
pkg/tarsum/tarsum.go:105: undefined: sort
```
```
105: sort.Strings(keys)
```