Create symbols packages, add serviceable attribute, and update assembly info (dotnet...

Create symbols packages, add serviceable attribute, and update assembly info (dotnet/core-setup#2331)

* Include copyright info, serviceable attribute, and produce symbols
packages for Microsoft.Extensions.DependencyModel and
Microsoft.DotNet.PlatformAbstractions

* Align AssemblyFileVersion with VersionPrefix, remove env vars from dotnet
pack

* Add license files

* small tweak to packagepath

* Add package licenseurl and projecturl

Commit migrated from https://github.com/dotnet/core-setup/commit/3542d52f0bd5ad6df84140120e954e43acd4b79a