db/simple/Directory: eliminate method LookupSong()

Move to SimpleDatabase::GetSong() to give that method more control.

src/db/plugins/simple/Directory.cxx

@@ -202,24 +202,6 @@ Directory::FindSong(const char *name_utf8) const
 	return nullptr;
 }
 
-Song *
-Directory::LookupSong(const char *uri)
-{
-	assert(holding_db_lock());
-	assert(uri != nullptr);
-
-	auto r = LookupDirectory(uri);
-	if (r.uri == nullptr)
-		/* it's a directory */
-		return nullptr;
-
-	if (strchr(r.uri, '/') != nullptr)
-		/* refers to a URI "below" the actual song */
-		return nullptr;
-
-	return r.directory->FindSong(r.uri);
-}
-
 static int
 directory_cmp(gcc_unused void *priv,
 	      struct list_head *_a, struct list_head *_b)

src/db/plugins/simple/Directory.hxx

@@ -213,17 +213,6 @@ public:
 		return const_cast<Song *>(cthis->FindSong(name_utf8));
 	}
 
-	/**
-	 * Looks up a song by its relative URI.
-	 *
-	 * Caller must lock the #db_mutex.
-	 *
-	 * @param uri the relative URI
-	 * @return the song, or nullptr if none was found
-	 */
-	gcc_pure
-	Song *LookupSong(const char *uri);
-
 	/**
 	 * Add a song object to this directory.  Its "parent" attribute must
 	 * be set already.

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -207,7 +207,25 @@ SimpleDatabase::GetSong(const char *uri, Error &error) const
 	assert(borrowed_song_count == 0);
 
 	db_lock();
-	const Song *song = root->LookupSong(uri);
+
+	auto r = root->LookupDirectory(uri);
+	if (r.uri == nullptr) {
+		/* it's a directory */
+		db_unlock();
+		error.Format(db_domain, DB_NOT_FOUND,
+			     "No such song: %s", uri);
+		return nullptr;
+	}
+
+	if (strchr(r.uri, '/') != nullptr) {
+		/* refers to a URI "below" the actual song */
+		db_unlock();
+		error.Format(db_domain, DB_NOT_FOUND,
+			     "No such song: %s", uri);
+		return nullptr;
+	}
+
+	const Song *song = r.directory->FindSong(r.uri);
 	db_unlock();
 	if (song == nullptr) {
 		error.Format(db_domain, DB_NOT_FOUND,