[confcom] Fix using `--infrastructure-svn` with additional fragments

[DomAyre]

If you run acipolicygen with both additional custom fragments, and set the minimum SVN of the infrastructure fragment. The policygen fails with:

Invalid artifact name: mcr.microsoft.com/aci/aci-cc-infra-fragment

[yonzhan]

Thank you for opening this issue, we will look into it.