[FIX] core: improve get_files performances

On a standard odoo install, `MigrationManager._get_files` represent
more than 4% of an install. This is because the legacy
odoo/base/maintenance/migration path was added to upgrade.__path__
once by module, making the get_filed check 574^2 os.path.exists.

This commit adds a check on initialize_sys_path to call it only once,
and merge legacy path with upgrade-path management in order to benefit
of the `up not in upgrade.__path__` check. This second part of the fix
will also remove the local dir from the upgrades paths.

A further improvement would be to fix MigrationManager in order to skip
_get_file work on a fresh install, (wip by rco-odoo)

closes odoo/odoo#45699

X-original-commit: 497330a695ed52a33f9c9b9c27b149446de0db29
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>

odoo/modules/module.py

@@ -32,6 +32,9 @@ def initialize_sys_path():
     Setup the addons path ``odoo.addons.__path__`` with various defaults
     and explicit directories.
     """
+    if getattr(initialize_sys_path, 'called', False): # only initialize once
+        return
+    initialize_sys_path.called = True
 
     # hook odoo.addons on data dir
     dd = os.path.normcase(tools.config.addons_data_dir)
@@ -51,16 +54,12 @@ def initialize_sys_path():
 
     # hook odoo.upgrade on upgrade-path
     from odoo import upgrade
-    for up in tools.config['upgrade_path'].split(','):
+    legacy_upgrade_path = os.path.join(base_path, 'base', 'maintenance', 'migrations')
+    for up in (tools.config['upgrade_path'] or legacy_upgrade_path).split(','):
         up = os.path.normcase(os.path.abspath(tools.ustr(up.strip())))
         if up not in upgrade.__path__:
             upgrade.__path__.append(up)
 
-    # hook odoo.upgrade on legacy odoo/addons/base/maintenance/migrations symlink
-    if not tools.config['upgrade_path']:
-        upgrade.__path__.append(os.path.join(
-            base_path, 'base', 'maintenance', 'migrations'))
-
     # create decrecated module alias from odoo.addons.base.maintenance.migrations to odoo.upgrade
     spec = importlib.machinery.ModuleSpec("odoo.addons.base.maintenance", None, is_package=True)
     maintenance_pkg = importlib.util.module_from_spec(spec)