[Bug 1813692] Re: Upgrade from 16.04 to 18.04 fails after disabling most third-party packages

[Michael Corey]

Thank you for your response. I have run

```
ppa-purge ubuntu-toolchain-r
```

Which generates:
```
Updating packages lists
PPA to be removed: ubuntu-toolchain-r ppa
Warning:  Could not find package list for PPA: ubuntu-toolchain-r ppa
```

However, I still get the same error when I try the distribution upgrade.
I don't see ubuntu-toolchain-r in my sources.list.

I also manually removed 3 files related to ubuntu-toolchain-r from my
/etc/apt/sources.list.d directory.

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https://bugs.launchpad.net/bugs/1813692

Title:
  Upgrade from 16.04 to 18.04 fails after disabling most third-party
  packages

Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid

Bug description:
  I have tried disabling any less-common third-party packages, but am getting the 
  "Could not calculate the upgrade" on the "Setting new software channels" step.

  I boot into Ubuntu via ReFind, if that makes any difference.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: ubuntu-release-upgrader-core 1:16.04.26
  ProcVersionSignature: Ubuntu 4.4.0-141.167-generic 4.4.162
  Uname: Linux 4.4.0-141-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.18
  Architecture: amd64
  CrashDB: ubuntu
  Date: Mon Jan 28 19:12:21 2019
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2016-10-10 (840 days ago)
  InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
  PackageArchitecture: all
  ProcEnviron:
   LANGUAGE=en_US
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to xenial on 2019-01-29 (0 days ago)
  VarLogDistupgradeTermlog:

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