Renamed NuGet package discovery classes

Since they are not Glimpse specific but are actually intended to
discover NuGet packages. The idea is to have the NuGetPackageAttribute
moved to NuGet eventually.
Aligned NuGetPackageAttribute values

- An explicit value of Glimpse for the Core projecten (otherwise we
would get Glimpse.Core as package name)
- An explicit value for the "normal/base" projects
- No value (so that it will be determined based on the assembly at
runtime) for the .NET framework version specific projects