Need to pass $(TargetExt) into GenerateDepsFile

[eerhardt]

We are currently hard-coding the assembly extension to .dll when generating the .deps.json file. We should be passing in $(TargetExt) instead and passing it through to SingleProjectInfo.

See

sdk/src/Tasks/Microsoft.NET.Build.Tasks/SingleProjectInfo.cs

Line 42 in e703af2

return $"{Name}.dll";