- storage/db/dbtest, storage/cmd/reindex: hide cloud SQL behind a build tag
  (run go test -tags cloud golang.org/x/perf/storage/db -cloud to run tests)
- analysis/appengine, storage/appengine: hide behind appengine build tag
  (automatically supplied by goapp)
- vendor: used govendor to import non-x non-appengine dependencies

Change-Id: Ia5590599e7191ab3427618204fab7d2cec907579
Reviewed-on: https://go-review.googlesource.com/36592
Run-TryBot: Quentin Smith <quentin@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>