Add --repo option to schema generator

[d3mcfadden]

I added a --repo option to the schema generator so apps that have multiple repos can have the ability to select what repo the schema/migration should use.

I noticed ecto.gen.migration supports this flag and assumed it would work in phoenix as well. It did not, so here it is :-)

This works on the schema generator and other "parent" generators that call it (eg: phx.gen.context)

NOTE to reviewer

Ideally, I wanted to support a custom priv config that may be set on the repo to determine the migration_path, however I noticed that will require me ensuring the repo module is compiled first. I wanted to get some advice on that before doing anything.

[josevalim]

I would rather support setting a custom migration directory and that's it. Changing the directory itself should be done manually IMO because you can end-up in scenarios where a context has multiple repos aliases as: Repo and that can be confusing and trying to address such corner use case through generators are not going to scale IMO. :)

[d3mcfadden]

@josevalim thanks for the feedback! That makes sense to me also. I'm working on an app that has 2 repos and manually have to move stuff around after using the generators. It is not that big of a deal, but I figured it may be useful to us and others.

I can update this to support --migration-path instead if you think that change would make it in. Otherwise, we can just close this out.

Thanks!

edit: would also throw out just removing the as: Repo stuff and letting the end author be responsible for doing that in the contexts.

[josevalim]

The migration path bit is welcome if you think it is helpful. Your call!

[josevalim]

Closing this./ If you want to send a PR for the custom migration path, it would be welcome!

[MikaAK]

I realised this was around after i created #4905 which I needed for a generator project I'm working on, I've implemented both though if you want me to strip one of the options lemme know