Fix default items (#1123)

There were some issues in the MSBuild evaluation order
that caused the default items files to never get used.

.nuspec/Xamarin.Forms.DefaultItems.props

@@ -1,6 +1,6 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
-	<ItemGroup>
+	<ItemGroup Condition="'$(EnableDefaultItems)'=='True' And '$(EnableDefaultXamlItems)'=='True' And '$(EnableDefaultEmbeddedResourceItems)'=='True'">
 		<EmbeddedResource Include="**\*.xaml" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" SubType="Designer" Generator="MSBuild:UpdateDesignTimeXaml" />
 	</ItemGroup>
 

.nuspec/Xamarin.Forms.DefaultItems.targets

@@ -7,7 +7,7 @@
 	Unfortunately there is no way to work around this without adding a new extension point to
 	the sdk props.
 	-->
-	<ItemGroup>
+	<ItemGroup Condition="'$(EnableDefaultItems)'=='True' And '$(EnableDefaultXamlItems)'=='True' And '$(EnableDefaultEmbeddedResourceItems)'=='True'">
 		<Compile Update="**\*.xaml$(DefaultLanguageSourceExtension)" DependentUpon="%(Filename)" SubType="Code" />
 		<None Remove="**\*.xaml" Condition="'$(EnableDefaultNoneItems)'=='True'" />
 	</ItemGroup>

.nuspec/Xamarin.Forms.props

@@ -10,6 +10,6 @@
 	The actual item groups are in a separate conditionally-imported file as they use constructs that
 	are not compatible with older MSBuild versions.
 	-->
-	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Forms.DefaultItems.props" Condition="'$(_DefaultXamlItemsEnabled)'=='True'" />
+	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Forms.DefaultItems.props" Condition="'$(MSBuildSDKsPath)'!=''" />
 
 </Project>

.nuspec/Xamarin.Forms.targets

@@ -6,10 +6,9 @@
 	<PropertyGroup>
 		<EnableDefaultXamlItems Condition="'$(EnableDefaultXamlItems)'==''">True</EnableDefaultXamlItems>
 		<_DefaultXamlItemsEnabled>False</_DefaultXamlItemsEnabled>
-		<_DefaultXamlItemsEnabled Condition="'$(EnableDefaultItems)'=='' And '$(EnableDefaultXamlItems)'=='True' And '$(EnableDefaultEmbeddedResourceItems)'=='True'">True</_DefaultXamlItemsEnabled>
 	</PropertyGroup>
 
-	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Forms.DefaultItems.targets" Condition="'$(_DefaultXamlItemsEnabled)'=='True'" />
+	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Forms.DefaultItems.targets" Condition="'$(MSBuildSDKsPath)'!=''" />
 
 	<PropertyGroup>
 		<CoreCompileDependsOn>