refactor(automigrate): Limit ordered.Map usage to migrator internals

[bevzzz]

This PR addresses one of the issues raised here:

One piece of feedback regarding ordered.Map as a dependency [...] The choice to use an internal type in exported fields of exported structs made this code unusable as is, and made extending and building upon it really really messy.

Indeed, Database, Table, and BunModelSchema (with Tables field) are all exported members of sqlschema package. Having them return internal/ordered.Map has several implications:

1. As noted in the comment, external packages aren't able to use return of Tables(), while other methods, e.g. ForeignKeys() return usable objects. Same is true for Columns().

2. Since sqlschema requires dialects to use/return ordered.Map, no otherwise compatible 3rd-party dialect will be able to implement sqlschema.Inspector

We've originally decided to use an ordered map to achieve stable query generation, which really is a great use case for it.

This PR limits the usage of the ordered map to where it's most useful -- the migrate.detector, which calculates schema diff. Meanwhile, schema inspectors will return Tables and Columns as slices, reducing the complexity and making the package more extendable and interoperable.

Some of the tests got a tiny bit more verbose as a result, but overall the code looks simpler.