image-minimizer: support drop-keep-drop
    
    Resolve removal set on the fly, so that drop-keep-drop can work.
    The order of drop*/keep* commands is important, last one wins!
    
    Example: given the file tree:
    d1/
    ├── d2/
    │   ├── d3/
    │   │   └── f
    │   └── f
    └── f
    
    and image-minimizer input:
    drop d1
    keep d1/d2
    drop d1/d2/d3
    
    it should keep only d1/d2:
    d1/
    └── d2/
        └── f
    
    Signed-off-by: Alan Pevec <apevec@redhat.com>