Magic Mode bootstrapper uses wrong download URL

Description

The paket bootstrapper doesn't seem to build correct package download URL when specifying a Azure DevOps Server as nuget-source.

Repro steps

In my paket.dependencies I specify a custom nuget source. It is a private feed of an Azure DevOps server.

version 6.2.1 --force-nuget --nuget-source=https://my-server/tfs/XXX/_packaging/BVMS/nuget/v3/index.json -v -v
...

I clear all the caches, then I run paket.exe --version
This fails. The output is as follows (some lines omitted at the beginning)

[Nuget] DownloadVersion...
Starting download from https://my-server/tfs/XXX/_packaging/BVMS/nuget/v3/index.json/package/Paket/6.2.1
[Nuget] DownloadVersion took 0,82 second(s) and failed with 'Version 6.2.1 wasn't found (404)'.
[Nuget - cached] DownloadVersion took 0,83 second(s) and failed with 'Version 6.2.1 wasn't found (404)'.
Version 6.2.1 wasn't found (404) (Nuget - cached)

Expected behavior

The correct URL in my case would be as follows:

https://my-server/tfs/XXX/_packaging/308ecb1e-187a-48db-b49f-2087453b7264/nuget/v3/flat2/Paket/6.2.1/paket.6.2.1.nupkg

It seems that the bootstrapper doesn't construct the URL correctly. I don't know how this can be derived, but paket itself does it correctly. I found out the correct URL by inspecting the verbose output of a paket install when downloading some other package.

Reproduce the failure with the shown paket.dependencies source and paket.exe --version, then compare the bootstrapper's URL construction with Paket's existing Azure DevOps NuGet handling. Done means the bootstrapper resolves the feed metadata and downloads Paket 6.2.1 from the correct flat2 package URL instead of the 404 URL.