NuGet: support building symbol+source packages without project file

NuGet supports building symbol/source packages automatically when specifying the project file. However, it is also possible to build such packages manually without a project file but instead a nuspec file, where the symbols and sources are specified explicitly. To my understanding this second approach without a project file is not currently supported with Fake.

The default behavior is unchanged: symbol packages are built if and only if a project file is provided (in this case we generate, as before, first both packages with the project file, then overwrite the normal package using the generated nuspec). If symbol packages should be built with the nuspec file, then specify p.SymbolPackage = NugetSymbolPackage.Nuspec.

I'm not very happy with this design, but it was the best I came up with that does not break existing behavior and does not require introducing a whole new set of NuGetXY-like functions. I happily update the PR if someone has a better idea. Thanks.