During import if files are found to be missing, ensure they are restored

This is one step closer to sanity in terms of physical files. As per the
comment I have left in place, we really should be checking file sizes or
hashes, but to keep things simple and fast I've opted to just cover the
"missing file" scenario for now.

Ran into this when testing against a foreign `client.realm` by:
- Noticing a beatmap doesn't load
- Deleting said beatmap
- Downloading via beatmap overlay
- Beatmap is restored but still doesn't work

Note that I've kept the logic where this will undelete an existing
import rather than create one from fresh, as I think that is beneficial
to the user (ie. it will still keep any linked scores on restore).

osu.Game.Tests/Database/BeatmapImporterTests.cs

@@ -600,6 +600,38 @@ namespace osu.Game.Tests.Database
             });
         }
 
+        [Test]
+        public void TestImportThenReimportAfterMissingFiles()
+        {
+            RunTestWithRealmAsync(async (realmFactory, storage) =>
+            {
+                using var importer = new BeatmapModelManager(realmFactory, storage);
+                using var store = new RulesetStore(realmFactory, storage);
+
+                var imported = await LoadOszIntoStore(importer, realmFactory.Context);
+
+                deleteBeatmapSet(imported, realmFactory.Context);
+
+                Assert.IsTrue(imported.DeletePending);
+
+                // intentionally nuke all files
+                storage.DeleteDirectory("files");
+
+                Assert.That(imported.Files.All(f => !storage.GetStorageForDirectory("files").Exists(f.File.GetStoragePath())));
+
+                var importedSecondTime = await LoadOszIntoStore(importer, realmFactory.Context);
+
+                // check the newly "imported" beatmap is actually just the restored previous import. since it matches hash.
+                Assert.IsTrue(imported.ID == importedSecondTime.ID);
+                Assert.IsTrue(imported.Beatmaps.First().ID == importedSecondTime.Beatmaps.First().ID);
+                Assert.IsFalse(imported.DeletePending);
+                Assert.IsFalse(importedSecondTime.DeletePending);
+
+                // check that the files now exist, even though they were deleted above.
+                Assert.That(importedSecondTime.Files.All(f => storage.GetStorageForDirectory("files").Exists(f.File.GetStoragePath())));
+            });
+        }
+
         [Test]
         public void TestImportThenDeleteThenImportNonOptimisedPath()
         {