[MSBuild] Fix search path evaluation issue

When evaluating an import for several different paths, make sure
the property assignments required for the different evaluations are
temporary, and don't have an effect on the global evaluation context.

Author: slluis

main/src/core/MonoDevelop.Core/MonoDevelop.Projects.MSBuild/DefaultMSBuildEngine.cs

@@ -988,7 +988,7 @@ string[] GetImportFiles (ProjectInfo project, MSBuildEvaluationContext context,
 			if (pathPropertyValue != null) {
 				var tempCtx = new MSBuildEvaluationContext (context);
 				var mep = MSBuildProjectService.ToMSBuildPath (null, pathPropertyValue);
-				tempCtx.SetPropertyValue (pathProperty, mep);
+				tempCtx.SetContextualPropertyValue (pathProperty, mep);
 				context = tempCtx;
 			}
 

main/src/core/MonoDevelop.Core/MonoDevelop.Projects.MSBuild/MSBuildEvaluationContext.cs

@@ -327,6 +327,12 @@ public void SetPropertyValue (string name, string value)
 				properties [name] = value;
 		}
 
+		public void SetContextualPropertyValue (string name, string value)
+		{
+			// Sets a properly value only for the scope of this context, not for the scope of the global evaluation operation
+			properties [name] = value;
+		}
+
 		public void ClearPropertyValue (string name)
 		{
 			properties.Remove (name);