cache/filecache: Add a :project placeholder

This allows for "cache per Hugo project", making `hugo --gc` work as expected, even if you have several Hugo projects running on the same PC.

See #5439

cache/filecache/filecache.go

@@ -272,7 +272,10 @@ func (c *Cache) getOrRemove(id string) hugio.ReadSeekCloser {
 }
 
 func (c *Cache) isExpired(modTime time.Time) bool {
-	return c.maxAge >= 0 && time.Now().Sub(modTime) > c.maxAge
+	if c.maxAge < 0 {
+		return false
+	}
+	return c.maxAge == 0 || time.Now().Sub(modTime) > c.maxAge
 }
 
 // For testing