[pack-images] Refactor for readability.

* Use classes instead of generators.
* Be explicit about what's a file path and what's a file contents rather
  than stuffing the file contents into a lambda.

Test: ninja -C out/x64 symbol-archive-tgz archive-tgz symbol-archive-zip archive-zip
Did this before and after my change. The only diff is the order of the
elements in build-archive/images.json.

Change-Id: Ida815c4ba314382500a06b1c329a99af012cec18
1 file changed