odoo_project: rename `odoo.project` to `odoo.project.instance`

[sebalix]

...and make odoo.project the entry point of an Odoo project, that will list the different odoo.project.instance, e.g.:

• production instance based on branch 16.0
• test/simulation instance based on 16.0 too (similar to production but with additional modules)
• migration instance based on 18.0

Depends on:

• Improve branches management for project's repository #67 (to clone the project repository only once, and fetch all the required branches in it)

cc @hparfr I think that's something you also talked about at OCA Days