import & skip statements combined leads to multiple repeated image edits

file1:
```ruby
from "alpine"
skip do
  run "ls"
end
```

file2:
```ruby
import "file1"
```

This will result in two layer editing series both editing in the same layers (we don't track multiple skip statements independently, the skipped layers are just kept in a list for editing at the end of the run).

This is painfully slow.