Define target frameworks/project locations in the common props/targets rather than the csproj itself (#2533)

* extract most of the target framework defintions out of the csproj

* remove every target framework mention from the sdk based csproj

* Move old style csprojes to use the projectversion too

* use better variable names

* use the nugetcoresrc and nugetclientssrc variables

test/TestExtensions/SampleCommandLineExtensions/SampleCommandLineExtensions.csproj

@@ -3,7 +3,7 @@
   <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
   
   <PropertyGroup>
-    <TargetFramework>net46</TargetFramework>
+    <TargetFramework>$(NETFXTargetFramework)</TargetFramework>
   </PropertyGroup>
   <PropertyGroup>
     <RootNamespace>SampleCommandLineExtensions</RootNamespace>