[Bug 1994702] [NEW] backportpackage with URL fails

[Readable Author]

Public bug reported:

```
user at hp-laptop:~$ backportpackage -d jammy -u ppa:readableauthor/ppa https://mxrepo.com/mx/repo/pool/main/i/i-nex/i-nex_7.6.0-0.1~mx19+1.dsc
Invalid URL 'https:/mxrepo.com/mx/repo/pool/main/i/i-nex/i-nex_7.6.0-0.1~mx19+1.dsc': No host supplied
```
Why does it report https:/ instead of https:// and how to fix it?

** Affects: ubuntu-dev-tools (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1994702

Title:
  backportpackage with URL fails

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