Allow missing assembly dependencies for components. (dotnet/core-setup#4751)

Allow missing assembly dependencies for components. (dotnet/core-setup#4751)

When resolving assembly dependencies for components, don't fail if the assembly doesn't exist on the disk (but it's still present in the .deps.json as a runtime asset).
For dynamically loaded components this behavior makes sense as such components are typically distributed as just the necessary pieces, with everything else provided by the app.

This is a temporary change mainly as a workaround for dotnet/sdkdotnet/core-setup#2660. Issue dotnet/core-setupdotnet/core-setup#4752 is tracking future need for revisiting this change.

Commit migrated from https://github.com/dotnet/core-setup/commit/ca431d0cefee116b5c4ca292961ab60ddd81611f