Fix bug #517974.

Fix bug #517974.
In class/Microsoft.Build.Tasks:

	* Makefile: Add System.Core .

In class/Microsoft.Build.Tasks/Microsoft.Build.Tasks:

	Fix bug #517974.
	* ResolvedReference.cs (AssemblyName): New.
	(IsPrimary): New. Primary (eg. specified in the project file) or
	dependent reference.
	(FoundInSearchPathAsString): New.
	* AssemblyResolver.cs (GetResolvedReference): Add a 'assembly name'
	param for the new field in ResolvedReference.
	(AssemblyNamesCompatible): Fix the check for public key token.
	Track api changes.

	* ResolveAssemblyReference.cs: Keep track of resolved assemblies,
	prevent duplicate references and other files.
	Process dependencies after resolving all the primary references.
	Search for dependencies in parent assembly's dir first and then
	the specified search paths.
	Detect assembly version conflicts and try to resolve.
	Improve logged messages.
	(PrimaryReference): New.
	(ResolveAssemblyReferenceHelper.AddUniqueFile): New extension method.

In tools/xbuild:

	* xbuild.Microsoft.CSharp.targets (Csc.References): Remove
	"@(ChildProjectReferences)" as it gets resolved earlier
	and is part of @(ResolvedFiles).

In tools/xbuild/tests:

	* standalone/Project01: Add references to two "external" assemblies
	with dependencies.
	* standalone/Project01/Program.cs: Add references in code to the new
	assemblies, to ensure that they are required for compilation.

git-svn-id: svn+ssh://mono-cvs.ximian.com/source/trunk/mcs@138329 e3ebcda4-bce8-0310-ba0a-eca2169e7518