`edgedb migration apply` reports missing migration files if files are symbolic links

- EdgeDB Version: 4.5+28216a1
- EdgeDB CLI Version: 4.1.0+03ae624
- OS Version: NixOS GNU/Linux 6.1.76

Steps to Reproduce:

1. copy migrations files anywhere and create symlinks to `migrations` directory
2. run `edgedb migrations apply --schema-dir=`

You'll se something like:
```
edgedb error: there is applied migration history in the database but "/nix/store/1g38wyrqdvg5pgx0bcsm953yf0kywrxw-faundr/api/dbschema/migrations" is empty
```

Start by reproducing `edgedb migrations apply --schema-dir=` with migration files copied elsewhere and symlinked into the `migrations` directory. Trace how the migration-apply command discovers and checks files when `--schema-dir` is used. Done means symlinked migration files are recognized and the command no longer reports the migrations directory as empty.

Written by the indexing model from the issue text.