Details:
- Move BSDiff (but not PagedArray) from namespace courgette to bsdiff.
- Change namespace courgette::qsuf to qsuf.
- Change bsdiff:search() to return struct {pos, size} so we don't need
  awkward pointer passing; update callers.
- Updated BSDiff callers. Also fix weird hybrid usage by setup_util.cc,
  which calls Courgette's BSDiff, but using OK from BSPatch.

BUG=608885

Review-Url: https://codereview.chromium.org/2031193002
Cr-Commit-Position: refs/heads/master@{#407924}