Fix output config retrieval for new outputs

This removes `output_find_config`, which would take the first matching
output config it found. This is fine if only a name output config,
identifier output config, or even just wildcard exist, but if there is
a name output config and identifier output config, they are not merged.
Instead, this introduces find_output_config, which is just a wrapper
for `get_output_config`. This ensures that both the name and identifier
output configs are respected.

This fixes the following case:
- For simplicity in this example, remove all output configs from config
- Run `swaymsg output <name> bg #ff0000 solid_color`
- Run `swaymsg output <identifier> scale 2`
- Disconnect and reconnect output

Without this, the output will have the background, but not the scale.
With this, the output will have both the background and scale

include/sway/output.h

@@ -88,8 +88,6 @@ struct sway_output *output_by_name_or_id(const char *name_or_id);
 
 void output_sort_workspaces(struct sway_output *output);
 
-struct output_config *output_find_config(struct sway_output *output);
-
 void output_enable(struct sway_output *output, struct output_config *oc);
 
 void output_disable(struct sway_output *output);