test suite now passes after OsPath conversion

The test suite was failing because of a bug in the Database/* modules.
I had replaced doesPathExist with doesDirectoryExist, but it was
checking the database file.

I have audited commit f1ba21d698 for
other changes to doesPathExist, and checked that doesDirectoryExist and
doesFileExist were used correctly.

The only change I found is in youtubeDl', where it used to return
directories that might have been created by youtube-dl. But it was
supposed to return media files, so changing it to use doesFileExist is
actually an improvement. Although only of theoretical benefit.

Note that it would actually be possible to keep using doesPathExist,
there is a version of that for OsPath as well. But the rest of these
changes seem safe.

Sponsored-by: Nicholas Golder-Manning

Database/Keys.hs

@@ -129,7 +129,7 @@ openDb forwrite _ = do
 	catchPermissionDenied permerr $ withExclusiveLock lck $ do
 		dbdir <- calcRepo' gitAnnexKeysDbDir
 		let db = dbdir </> literalOsPath "db"
-		dbexists <- liftIO $ doesDirectoryExist db
+		dbexists <- liftIO $ doesFileExist db
 		case dbexists of
 			True -> open db False
 			False -> do