Incorrect assembly resolved in mixed target environements

### Describe the bug
ResolvePackageAssets may resolve incorrect assembly from NuGet in builds where top and dependent project has different TFM targets, top level project nuget TFM runtime assemblies are empty, but dependent TFM assemblies are non empty.

### To Reproduce
Build/publish app/app.csproj from https://gist.github.com/iskiselev/cbd061c782d5333dc284826c3f3ef791

### Further technical details
In specific example, we have next dependency structure:
app.csproj (net471) -> lib.csproj (net462) ->System.ValueType/4.6.1

NuGet resolution should be done in target project TFM. `System.ValueType/4.6.1` has `lib\net471\_._`, which means that no files should be copied to output from nuget package in `net471`. But output folder will have System.ValueType.dll from `lib\net462`. It happens only when lib.dll has reference to System.ValueType.dll (I've not validated behaviour with transitive dependencies over nuget). In that case ResolvePackageAssets will pick up System.ValueType.dll that have been copied to lib.csproj bin folder.