Allow arbitrary packs

Summary:
The limitation to two types is arbitrary and limiting. Instead, we want to be able to create as many cache artifacts as is sensible to improve the caching behaviour.

A lot of unnecessary cloning in here. I might optimise this a bit in the future but it's not really perf critical as this is highly I/O bound.

Reviewed By: lblasa

Differential Revision: D38155922

fbshipit-source-id: 78b86ebff54269c5049e59197f1c25fedfad0111

packer/src/packlist.yaml

@@ -50,3 +50,7 @@ server-linux:
       - node
     core:
       - '!node'
+      - '!static'
+    static:
+      - '!*'
+      - static