Private repo causes transient extra dependencies to be tossed #2300

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Windows 10 1909 x64
• Poetry version: 1.0.5
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/madig/de961d21c8f796b4fc86d984dece0fd4

Issue

The presence of a secondary package index and a package from there as a dependency messes with dependency resolution.

To reproduce:

1. poetry new test && cd test
2. Insert content of https://gist.github.com/madig/de961d21c8f796b4fc86d984dece0fd4 into pyproject.toml
3. poetry install -- note how this installs appdirs, fs, pytz and six.
4. poetry update -- removes these packages again when it shouldn't.
5. Comment out the test repository and the poetry-resolver-bug-test package
6. poetry update reinstalls appdirs and friends
7. Reenable repo and package
8. poetry update -- installs the poetry-resolver-bug-test package but leaves appdirs, etc. alone
9. Redoing poetry update then tosses them again when it shouldn't.

In my actual package, the toml package (via isort[pyproject]) also gets tossed out, but I can't reproduce this here easily.