Add --update-list CLI option

[dreispt]

CLI option to update the module list.
Using it allows to directly install newly added modules without going to the web GUI or updating the base module.

[mart-e]

Hello,

This option will only work if you are in update mode (e.g. install another module).
Either it is explicitly specified in the command line (anyway we will usually do -U -i my_new_module), either we activate update_mode with the -U (but I am afraid activating the update mode would have more side effects).

Otherwise looks good for master

[dreispt]

Should the help text mention it should be used with -d and -i?
Should I make a PR for master then?

[xmo-odoo]

Alternatively, wouldn't it make sense to update the modules list any time we're trying to install a new module? /cc @odony

[odony]

Yes, we already have so many CLI parameters, it'd be much more in favor of merging this feature with the update/init mode itself. The likely reason why we didn't do that yet is the (small) extra cost of the addons_path scanning. But I think it would be ok to do it at the beginning of every install and/or update, not just for base (and in master, not a in stable version).

[xmo-odoo]

I went hunting for the origin of this expecting a revision 1 introduction but it turns out to be 00098e7: the restriction to "only when installing or updating base" seems somewhat arbitrary and is not explained.

[dreispt]

That's even simpler. I can do that PR if you like.

[dreispt]

PR made here: #9140