Provide way to disable trimming against shared framework for source-build installations

[natemcmaster]

To address dotnet/source-build#456, we want to make it easier to opt-out of the aspnet shared framework.

Changes to make in our code

• Make the override for MicrosoftNETPlatformLibrary in our .App and .All packages conditional on a property.
• Set this property in source-build installations of the SDK

Implications

• Default templates will build and run on source-build installations without requiring changes to the file or special properties passed as command line flags.
• Apps will run from the NuGet cache, not the optimized shared framework. This means slower startup and larger application size.
• Users who publish an app with a non-source-build SDK (e.g. publish on Windows) and then try to run on a source-build-only server will be missing the shared framework. This situation is similar to several other scenarios where servers may not be configured with the aspnet shared framework, so we believe this will be address with documentation on how to configure a server with the aspnet shared runtime.

cc @omajid @tmds @muratg

[natemcmaster]

Done. Changes are in to both dotnet/CLI and aspnetcore.

#1130
dotnet/cli#9178