Refactor DllImport load functions

This change refactors the code in DllImport in preparation
for implementing the new  NativeLibrary API here:
https://github.com/dotnet/corefx/issues/32015

In particular, it introduces a change in the semantics of the
internal LoadLibrary helper functions.

When a native library is loaded, there are two categories of callers
expecting different return values:
 * External callers likeAssemblyNative::InternalLoadUnmanagedDllFromPath()
   and the upcoming System.Runtime.Interop.Marshall.LoadLibrary()
   need the raw system handle
 * Internal callers like LoadLibraryModule() need the PAL registered handle

This change modifies the internal LoadLibraryModule* methods to work
in terms of native system handles, so that external callers can obrain
them directly. Methods requiring PAL-handles can register them explicitly.

There is no change in external signature of DllImport class, or the
native Dll cache in AppDomain class.