Paket convert-from-nuget fails when package source keys contain invalid XML element chars

[sneal]

Running paket convert-from-nuget fails with error:

paket convert-from-nuget -v
Paket version 0.6.9.0
Paket failed with:
   '//packageSourceCredentials/DaptivBus @ AppVeyor' has an invalid token.
StackTrace:
     at MS.Internal.Xml.XPath.QueryBuilder.Build(String query, Boolean allowVar, Boolean allowKey)
   at System.Xml.XPath.XPathExpression.Compile(String xpath, IXmlNamespaceResolver nsResolver)
   at System.Xml.XPath.XPathNavigator.Select(String xpath)
   at System.Xml.XmlNode.SelectNodes(String xpath)
   at [email protected](IEnumerable`1& next)
   at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase`1.MoveNextImpl()
   at Microsoft.FSharp.Collections.SeqModule.ToList[T](IEnumerable`1 source)
   at Paket.NuGetConvert.convertNugetsToDepFile(IEnumerable`1 nugetPackagesConfigs)
   at Paket.NuGetConvert.ConvertFromNuget(Boolean force, Boolean installAfter, Boolean initAutoRestore)
   at [email protected](Unit unitVar)
   at <StartupCode$paket>.$Paket.Program.main@()

When given the following NuGet.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <solution>
    <add key="disableSourceControlIntegration" value="true" />
  </solution>
  <packageSources>
    <clear />
    <add key="DaptivBus @ AppVeyor" value="https://ci.appveyor.com/nuget/daptiv-bus-15dvq7rlh05a" />
  </packageSources>
  <activePackageSource>
    <add key="DaptivBus @ AppVeyor" value="https://ci.appveyor.com/nuget/daptiv-bus-15dvq7rlh05a" />
  </activePackageSource>
</configuration>

Changing the package source key from DaptivBus @ AppVeyor to DaptivBusAtAppVeyor works around the issue.

[forki]

/cc @theimowski is this fixable?