Reorganize DLL imports

[jkotas]

corefx switched back to PInvokes using classic Windows .dll names: dotnet/corefx#13440

We should do the same for CoreRT.

[morganbr]

We need to understand how this will affect using these libraries on Windows Server Core and in UWP applications.

[jkotas]

ProjectN is running with CoreFX bits that link against kernel32.dll already. There should not be a problem with doing this cleanup in CoreRT.

[morganbr]

I'm not sure those bits have shipped... I'd suggest confirming whether P/Invokes for kernel32 pass WACK and work on all of the OSes ProjectN supports, including all supported versions of Windows 10 desktop, Phone, XBox, HoloLens, and IoT.

[jkotas]

@yizhang82 have investigated this before corefx switched over, and concluded that we should be ok. We would not have switched corefx over otherwise.