Option to generate schemas as separate packages

[sgtsquiggs]

What do you want to change?

I'd love to be able to generate several schemas at once and have them use separate go packages instead of prefixing the types:

Instead of models.SchemanameTablename generate schemaname.Tablename. This should alleviate any potential naming conflicts without having to prefix every type.

A check prior to generation could detect circular dependencies and fail with helpful error messaging instead.

What database engines need to be changed?

PostgreSQL, MySQL, SQLite

What programming language backends need to be changed?

Go

[kyleconroy]

While not the same, this is also related to #1941. Once we have the ability to split models into separate packages, we could then split out the schemas into separate packages as well.