Declaring dependency on packages from alternate sources

I'd like to be able to specify in the nuspec a dependency on a package from one of the alternate sources, in particular WindowsFeatures or WebPI.

For example:

```

```

Start by reviewing how nuspec dependency declarations are parsed and how package sources are selected. Use the WindowsFeatures and WebPI examples as the expected input, and verify that dependencies declaring either source are resolved from that alternate source rather than the default package source.

Written by the indexing model from the issue text.