Add support for Singleton's in VocabularyAnnotationInaccessibleTarget Rule.

[marabooy]

Short summary (3-5 sentences) describing the issue.

Currently adding a new out of line annotation for singletons causes the VocabularyAnnotationInaccessibleTarget rule to fail. According to the OData spec singletons are uniquely identified and shoulc be able to be addressed using a unique target consutructed using the rule MySchema.MyEntityContainer/MySingleton

Assemblies affected

Assemblies affected 7.*

Reproduce steps

  var model = CsdlReader.Parse(XmlReader.Create(@"C:\dev\Example.csdl"));
            model.Validate(out var errors);
            foreach (EdmError error in errors)
            {
                Console.WriteLine(error.ErrorMessage + error.ErrorLocation);
            }
         

Using the following csdl

<?xml version="1.0" encoding="utf-8"?>
<edmx:Edmx Version="4.0" xmlns:edmx="http://docs.oasis-open.org/odata/ns/edmx">
    <edmx:DataServices>
        <Schema Namespace="Example" xmlns="http://docs.oasis-open.org/odata/ns/edm">
            <EntityType Name="Person" OpenType="true">
                <Key>
                    <PropertyRef Name="UserName" />
                </Key>
                <Property Name="UserName" Type="Edm.String" Nullable="false">
                </Property>
                <Property Name="FirstName" Type="Edm.String" Nullable="false" />
            </EntityType>
            <EntityContainer Name="DefaultContainer">
                <Singleton Name="Me" Type="Example.Person">
                </Singleton>
            </EntityContainer>
            <Annotations Target="Example.DefaultContainer/Me">
                <Annotation Term="Org.OData.Core.V1.Description" String="Represents self."/>
            </Annotations>
        </Schema>
    </edmx:DataServices>
</edmx:Edmx>

Expected result

No errors are produced.

Actual result

The entity set 'Me' could not be found.(18, 18)