Switch to using PackageLicenseExpression

[lahma]

• license expression is the preferred way (shows in NuGet.org and used in license validations)
• added /p:ContinuousIntegrationBuild=True to get deterministic build (see warning https://nuget.info/packages/CommandLineParser/2.9.1)
• added <EmbedUntrackedSources>true</EmbedUntrackedSources> to remove warning (see warning https://nuget.info/packages/CommandLineParser/2.9.1)
• moved packed items to same section

Now all warnings gone and proper license expression present when packing locally: