Fix(plugin): deduplicate plugin directories list

[Mary-Clb]

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes !42478
• Prevents duplicate plugin directory processing and related side effects (such as duplicate hook/search option registrations) when directory entries are repeated.

[Rom1-B]

There are several places where GLPI_PLUGINS_DIRECTORIES is called directly without going through this function.

[trasher]

It sounds strange there are duplicates in this kind of constant definition. If issue is confirmed; it probably must be fixed at the declaration.

[Rom1-B]

By default, we have:

'GLPI_MARKETPLACE_DIR' => $this->root_dir . '/marketplace',
'GLPI_PLUGINS_DIRECTORIES' => [
    '{GLPI_MARKETPLACE_DIR}',
    $this->root_dir . '/plugins',
],

However, if in locale_define.php it is written:

define('GLPI_MARKETPLACE_DIR', GLPI_ROOT . '/plugins');

GLPI_PLUGINS_DIRECTORIES has the same value twice.

[trasher]

In this case the local_define is bad.
But anyway; rather than trying to dedup everywhere the constant is used, the fix must be done at its instanciation.