Release upgrade disco->eoan fails when Ubuntu mirrors are specified in mirrors:// format

[Philipp Reichmuth]

When updating Ubuntu 19.04 (disco) to 19.10 (eoan), ubuntu-release-upgrader throws the following error about missing package information and a missing ubuntu-minimal-package (in German):

```
Ungültige Paketinformationen

Das benötigte Paket »ubuntu-minimal« wurde nach der Aktualisierung
Ihrer Paketinformationen nicht gefunden. Dies kann daran liegen, dass
Sie keine offiziellen Spiegel-Server in Ihren Paketquellen
eingetragen haben oder ein Spiegel-Server überlastet ist.
Ihre derzeit eingerichteten Paketquellen finden Sie in
/etc/apt/sources.list. Im Falle eines überlasteten Spiegel-Servers
können Sie es später erneut versuchen.
```

In my `/etc/apt/sources.list`, the Ubuntu mirrors are specified through `mirrors.txt` and `mirrors://` URLs, as in

```
deb mirror://mirrors.ubuntu.com/mirrors.txt/ disco main restricted
...
```

When I change that manually to a regular HTTP URL for some fixed mirror (e.g. `deb http://de.archive.ubuntu.com/ubuntu/ disco main restricted`), it works and the upgrade process proceeds normally.

It looks like ubuntu-release-upgrader does not recognize the `mirrors://` URL format when the package sources from disco to eoan. As a result during the upgrade process the sources list ends up almost empty (only InRelease packages), and consequently the ubuntu-minimal package is not found.

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: ubuntu-release-upgrader-core 1:19.04.16.7
ProcVersionSignature: Ubuntu 5.0.0-31.33-generic 5.0.21
Uname: Linux 5.0.0-31-generic x86_64
ApportVersion: 2.20.10-0ubuntu27.1
Architecture: amd64
CrashDB: ubuntu
Date: Sat Oct 26 10:52:49 2019
InstallationDate: Installed on 2018-12-26 (304 days ago)
InstallationMedia: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to disco on 2019-10-26 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2019-10-26 10:52:28.812605
 Log time: 2019-10-26 10:52:35.715386
 Log time: 2019-10-26 10:52:49.543592
 Log time: 2019-10-26 10:52:51.766789

[Balint Reczey (rbalint)]

Also affecting Bionic-> Eoan upgrades and will affect Bionic -> Focal when Focal becomes stable.