Make sure package targets netstandard2.0

[kzu]

Since the package is just build targets, it ends up as a package that
doesn't target any specific target framework. This is problematic
for nuget restore, which won't bring anything from this when it's
referenced transitively from a TF-targeting package (i.e. Merq).

So, make sure our targets also target NS2, so the package is flagged
as TF-specific too (although general enough API-wise to target anything
MS.Extensions.DI targets too.