During the conversion of installer migration files (Pull Request: EZP-31309: Switch to Ibexa Migrations bundle for install process) it became clear that migration files, remaining in vendor directory (vendor/ibexa/installer/src/bundle/Resources/install/migrations), will need to be imported into migration bundle configured migration file directory (src/Migrations/Ibexa by default).

Migrations are supposed to be a kind-of "source code" for database data, so they should be included in the application code. Therefore we could:
1. Allow adding multiple directories into migration bundle as sources, including vendor directory.
2. Add some sort of import command / option that will copy the files from vendor directory into configured migration directory.
3. Make install command copy migration files into proper migration directory.
4. Allow executing .yaml files from any location when calling migration command.

This also should include a clear instruction / explanation for errors during migration command execution to allow developers to more easily solve problems occuring when calling the command with migration files outside of configured migration directory.