Go through the unconfigured project to get the configured project

We're trying to go from an `IVsBrowseObjectContext` to an `IPackageReferencesService`.
We were doing this through `IVsBrowseObjectContext.ConfiguredProject`, but it turns out
that this property always returns null on the particular implementation of
`IVsBrowseObjectContext` we're working with.

Instead, we need to go through the `UnconfiguredProject` and ask it for the "suggested"
`ConfiguredProject`.

I've also switched from `GetAwaiter().GetResult()` to properly async/await code inside
an `ExecuteSynchronously` call, mostly for the purposes of readability.

src/VisualStudio/IntegrationTest/TestUtilities/InProcess/SolutionExplorer_InProc.cs

@@ -221,9 +221,13 @@ public void AddPackageReference(string projectName, string packageName, string v
 
             if (project is IVsBrowseObjectContext browseObjectContext)
             {
-                var packageService = browseObjectContext.ConfiguredProject.Services.PackageReferences;
+                var threadingService = browseObjectContext.UnconfiguredProject.ProjectService.Services.ThreadingPolicy;
 
-                var result = packageService.AddAsync(packageName, version).GetAwaiter().GetResult();
+                var result = threadingService.ExecuteSynchronously(async () =>
+                {
+                    var configuredProject = await browseObjectContext.UnconfiguredProject.GetSuggestedConfiguredProjectAsync();
+                    return await configuredProject.Services.PackageReferences.AddAsync(packageName, version);
+                });
             }
             else
             {
@@ -237,9 +241,13 @@ public void RemovePackageReference(string projectName, string packageName)
 
             if (project is IVsBrowseObjectContext browseObjectContext)
             {
-                var packageService = browseObjectContext.ConfiguredProject.Services.PackageReferences;
+                var threadingService = browseObjectContext.UnconfiguredProject.ProjectService.Services.ThreadingPolicy;
 
-                packageService.RemoveAsync(packageName).GetAwaiter().GetResult();
+                threadingService.ExecuteSynchronously(async () =>
+                {
+                    var configuredProject = await browseObjectContext.UnconfiguredProject.GetSuggestedConfiguredProjectAsync();
+                    await configuredProject.Services.PackageReferences.RemoveAsync(packageName);
+                });
             }
             else
             {