Don't allow equal query and schema paths

[dominikbraun]

At the moment (without PR #178), this config file causes sqlc to panic:

{
  "version": "1",
  "packages": [
    {
      "name": "db",
      "emit_json_tags": true,
      "emit_prepared_queries": false,
      "path": "db",
      "queries": "db.sql",
      "schema": "db.sql"
    }
  ]
}

sqlc can't handle the situation where the queries value is equal to the schema value.

While #178 fixes the panic from a technical point of view and prevents sqlc from panicking in other situations, this PR checks if the paths are the same and informs the user if necessary.

[kyleconroy]

See #180 for an alternative to this change

[dominikbraun]

I'd prefer #180 since it allows the same path/file.

[kyleconroy]

Great, closing this in favor of merging #180