Refactor merged field structure

Here is the new model:

+-----------------------------+
|          Structure          |
+-----------------------------+
| +createFromDataboxes()      |
| getAllFields()              |
| getUnrestrictedFields()     |
| getPrivateFields()          |
| getFacetsFields()           |
| getThesaurusEnabledFields() |
| getDateFields()             |
|- - - - - - - - - - - - - - -|
| add()                       |
| get()                       |
| typeOf()                    |
| isPrivate()                 |
+-------+-+-+-----------------+
        | | |          +---------------------+
        | | +--------> |        Field        |
        | |            +---------------------+
        | |            | getName()           |
        | |            | getType()           |
        | |            | isXXX()             |
        | |            | getThesaurusRoots() |
        | |            +---------------------+
        | |
        | |            +-------+
        | +----------> | Field |
        |              +-------+
        |
        |              +-------+
        +------------> | Field |
                       +-------+

It was driven by the following use cases:
- Get list of facets (only searchable fields)
- Get list of fields with concept inference
- Get list of all fields
    - Splitted in private / public fields (to define mapping)
- Get all date fields
- Get field type
    - To apply sanitization rules
    - To define mapping
- Check if concept inference enabled
- Check if the field is searchable
- Check if the field is a facet
- Check if the field is private
- Dereference field from label (still to be done)

(The last two UCs are new)

Also removed old code from legacy search engines.

[#PHRAS-500]

bin/console

@@ -122,15 +122,13 @@ $cli->command(new H264MappingGenerator());
 $cli->command(new XSendFileConfigurationDumper());
 $cli->command(new XSendFileMappingGenerator());
 
-if ($cli['search_engine.type'] === SearchEngineInterface::TYPE_ELASTICSEARCH) {
-    $cli->command(new IndexCreateCommand());
-    $cli->command(new IndexDropCommand());
-    $cli->command(new MappingUpdateCommand());
-    $cli->command(new IndexPopulateCommand());
-    $cli->command(new QueryParseCommand());
-    $cli->command(new QuerySampleCommand());
-    $cli->command(new FindConceptsCommand());
-}
+$cli->command(new IndexCreateCommand());
+$cli->command(new IndexDropCommand());
+$cli->command(new MappingUpdateCommand());
+$cli->command(new IndexPopulateCommand());
+$cli->command(new QueryParseCommand());
+$cli->command(new QuerySampleCommand());
+$cli->command(new FindConceptsCommand());
 
 $cli->loadPlugins();