Allow 'migrations' folder as subfolder of `sql`, not `sql/sql_server`

[schuemie]

In recent versions of SqlRender it is allowed to put your OhdsiSql in the inst/sql folder instead of the inst/sql/sql_server folder, and loadRenderTranslateSql() will still work. The main reasons are

1. Techincally, it's OhdsiSql, not SQL Server SQL,
2. Looks prettier.
3. SqlRender has proven to be very capable of handling cross-platform idiosyncrasies, so most (if not all) packages have no need for platform-specific SQL.

(Note that plafform-specific code can still be placed in the appropriately named subfolders. It is just that 99.9% of SQL is OhdsiSql, so why not put it up a level?)

Currently ResultModelManager throw an error if I put the 'migrations' folder in the 'sql' folder. A simple extra lookup when looking for migration SQL would solve this. Could this be added?

[azimov]

Because of the support for flat folder structures outside of a package this issue is not as straightforward to resolve as I would have liked, I will add this in a future version when I have a bit more time to test it fully.

[schuemie]

Just a reminder that it would still be nice to have this