handle hierarchical fetch

[plicease]

nasm is provided on their dist dir in a hierarchical form

1.10.1/nasm-1.10.1.tar.gz
1.12.1/nasm-1.12.1.tar.gz
...

or what have you. This isn't handled by Alien::Build core plugins, but it can be done from an alienfile now. Would be a good place to demo the technique. I can provide some pointers for someone who wants to try their hand at this. Otherwise I will get to it when I get to it.