The SolutionDir property is only available when building from the
solution. For packing, it's actually not necessary and we could
also easily allow packing from the Moq.csproj project itself.

This change, in addition to enabling that, also makes the icon
path more flexible since it could be moved down to the src folder
(say) without requiring a change to the .csproj.

Since we're generating portable symbols, these are WAY smaller than
full PDBs used to be. Their size contribution to the package are
negligible and for convenience, it's more common nowadays to just
embed the symbols always in the assembly itself. In particular for
a library that is not deployed to production apps, this makes
perfect sense.

So stop generating the (effectively legacy) .snupkg.