Only enable MEF services for named process

When current process is different to named process, set the MEF
export's contract name to "GitHub.Disabled". Since this contract name
isn't imported, the service is effectively disabled.

Author: jcansdale

src/GitHub.Exports/Exports/ExportForProcess.cs

@@ -14,19 +14,24 @@ namespace GitHub.Exports
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property | AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = true)]
     public sealed class ExportForProcessAttribute : ExportAttribute
     {
+        // Unique name for exports that have been disabled
+        const string DisabledContractName = "GitHub.Disabled";
+
         /// <summary>
         /// Define an export that is only exposed in a specific named process.
         /// </summary>
         /// <param name="contractType">The contract type to expose.</param>
         /// <param name="processName">Name of the process to expose export from (e.g. 'devenv').</param>
-        public ExportForProcessAttribute(Type contractType, string processName) : base(ExportForProcess(contractType, processName))
+        public ExportForProcessAttribute(Type contractType, string processName) :
+            base(ContractNameForProcess(contractType, processName), contractType)
         {
             ProcessName = processName;
         }
 
-        static Type ExportForProcess(Type contractType, string processName)
+        static string ContractNameForProcess(Type contractType, string processName)
         {
-            return Process.GetCurrentProcess().ProcessName == processName ? contractType : null;
+            var enabled = Process.GetCurrentProcess().ProcessName == processName;
+            return enabled ? null : DisabledContractName;
         }
 
         /// <summary>