This commit removes the direct use of `pkg/distro/fedora` and
uses `distrofactory` instead. This is the more generic interface,
it also helps with PR#1563 as `pkg/distro/fedora` goes away.
Note that I did not test this but @bcl was kind enough to
mention how this is used:
```
The output from this is used in Test_upgrade in
internal/store/json_test.go, the files there are manually
created when we change the format of the store and add a new
parser to internal/store/json.go -- which hasn't happened in
a long time :)
```
I updated the cmd/README with a snippet of the above.
