build: switch to slog Handlers

Start using the new slog handlers. With this commit we also remove the
need for the LogWriter since we let our LogRotator implement io.Writer
and pass that in to our log file handler.

build/handler_sets.go

@@ -175,3 +175,27 @@ func (r *reducedSet) WithGroup(name string) slog.Handler {
 
 // A compile-time check to ensure that handlerSet implements slog.Handler.
 var _ slog.Handler = (*reducedSet)(nil)
+
+// subLogGenerator implements the SubLogCreator backed by a Handler.
+type subLogGenerator struct {
+	handler btclog.Handler
+}
+
+// newSubLogGenerator constructs a new subLogGenerator from a Handler.
+func newSubLogGenerator(handler btclog.Handler) *subLogGenerator {
+	return &subLogGenerator{
+		handler: handler,
+	}
+}
+
+// Logger returns a new logger for a particular sub-system.
+//
+// NOTE: this is part of the SubLogCreator interface.
+func (b *subLogGenerator) Logger(subsystemTag string) btclog.Logger {
+	handler := b.handler.SubSystem(subsystemTag)
+
+	return btclog.NewSLogger(handler)
+}
+
+// A compile-time check to ensure that handlerSet implements slog.Handler.
+var _ SubLogCreator = (*subLogGenerator)(nil)