Decoupling loco-cli from project for db related commands.

[HiImJulien]

Description

Sometimes, the project is in a state, in which the project does not compile. Maybe you were to eager and implemented a function against a entity, that does not exist yet. Maybe a migration has not run yet or you've broken your project like described below.

If in such a state, running cargo loco fails, because the main project does not compile, making you unable to run migrations from loco-cli.

To Reproduce

I've had this idea because I broke my project by accident:

1. Create a new database.
2. Forget to run the initial migrations.
3. Run cargo loco db entities.
4. Et voilá! Your project is in a defunct state. Migrations and entities cannot be generated.

Expected Behavior

Since the migrations are stored in a separate project, it'd be great if we could run them from loco-cli even though the main project does not compile, without having to run migration executable manually and creating the missing entities manually.

Environment:

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Additional Context

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