Allow separate dependencies for buggy and fixed versions

[ChrisTimperley]

Tag buggy version as a layer in the fixed image.

Plan

build-rosinstall.py: should construct a .rosinstall file for the dependencies of the buggy version of the PUTs, named deps.pre.rosinstall. build-rosinstall.py should also create a separate .rosinstall file for the dependencies of the fixed version of the PUTs, named deps.post.rosinstall. To avoid increasing build times and inflating the image size by potentially rebuilding the same dependencies, deps.post.rosinstall will only store packages whose versions do not appear in deps.pre.rosinstall.

Changes

To support this new functionality, we will need to modify the format of the .bug file. Specifically, we will need to specify a separate datetime for the bug and the fix. It's also unclear whether issues can still be used to avoid the need to specify timestamps.

Store two lists of missing dependencies: one for the buggy version, and another for the fixed version.