The plugin docs tell plugins to implement Migrations() returning
[]*xormigrate.Migration, but neither `xorm.io/xorm` nor
`src.techknowlogick.com/xormigrate` was in the yaegi symbol table, so an
interpreted plugin failed at the import before it could ever be loaded.
Add both packages to yaegiSymbolPackages and commit the generated symbol
tables, plus a loader test that runs an interpreted plugin migration
against a real engine.
Fixes#3501