Made RemoveUnnecessaryPackages consistent (#696)

In the old code it would look for nuspec files on the file system and then
remove folders that are not in packages.config. But installedPackages
already contain all found nuspec files. The issue with old code is that
InstalledPackages contained data about packages that were also
potentially modified by the plugins while this method didn't take that
into account leading to inconsistencies.

src/NuGetForUnity/Editor/InstalledPackagesManager.cs

@@ -246,7 +246,7 @@ internal static void UpdateInstalledPackages()
         }
 
         /// <summary>
-        ///     Checks if there are any packages inside the package install directory that are not listed inside the packages.config.
+        ///     Finds and removes any installed packages detected that are not listed inside the packages.config.
         /// </summary>
         /// <returns>True if some packages are deleted.</returns>
         internal static bool RemoveUnnecessaryPackages()
@@ -256,38 +256,17 @@ internal static bool RemoveUnnecessaryPackages()
                 return false;
             }
 
-            var directories = Directory.GetDirectories(ConfigurationManager.NugetConfigFile.RepositoryPath, "*", SearchOption.TopDirectoryOnly);
             var somethingDeleted = false;
-            foreach (var folder in directories)
+            foreach (var installedPackage in InstalledPackages)
             {
-                var folderName = Path.GetFileName(folder);
-                if (folderName.StartsWith(".", StringComparison.Ordinal))
-                {
-                    // ignore folders whose name starts with a dot because they are considered hidden
-                    continue;
-                }
-
-                var nuspecPath = Directory.GetFiles(folder, "*.nuspec").FirstOrDefault();
-                if (!File.Exists(nuspecPath))
-                {
-                    // ignore folder not containing a nuspec file
-                    continue;
-                }
-
-                var package = NugetPackageLocal.FromNuspecFile(
-                    nuspecPath,
-                    new NugetPackageSourceLocal(
-                        "Nuspec file already installed",
-                        Path.GetDirectoryName(nuspecPath) ?? throw new InvalidOperationException($"Failed to get directory from '{nuspecPath}'")));
-
-                var installed = PackagesConfigFile.Packages.Exists(packageId => packageId.Equals(package));
+                var shouldBeInstalled = PackagesConfigFile.Packages.Exists(packageId => packageId.Equals(installedPackage));
 
-                if (!installed)
+                if (!shouldBeInstalled)
                 {
                     somethingDeleted = true;
-                    NugetLogger.LogVerbose("---DELETE unnecessary package {0}", folder);
+                    NugetLogger.LogVerbose("---DELETE unnecessary package {0}", installedPackage.Id);
 
-                    PackageContentManager.DeletePackageContentPackage(package);
+                    PackageContentManager.DeletePackageContentPackage(installedPackage);
                 }
             }