Add Buildstream compatibility

[Qinusty]

This issue will encompass the work required to bring Buildbarn into a state which it can work with Buildstream in accordance with the REAPI spec.

Buildbarn Issues:

• Buildbarn creates output directories based on the root directory #18: This needs to be resolved as it is not REAPI compliant and causes issues with clients which choose to use working directories other than the input root.
• Move input root to sub directory of build directory #20: This is likely required to allow local_build_executor.go to provide the input root as an output directory.
  • Alternatively, bb-storage could be altered to allow local_directory to accept "." as a filepath component to Lstat to allow root to be selected for Lstat whilst the outputParentDirectory is root.
• bb_runner: Allow it to chroot into the input root #23: This could be useful to allow clients to provide their own toolchains and avoid using toolchains provided by the runner.

Following the implementation of fixes for these two issues, I predict that Buildbarn should work with Buildstream as long as the runner executes the action with the input root as the root through some form of sandboxing.

[EdSchouten]

Is there still anything that needs to be done here?

[Qinusty]

All of the immediate issues mentioned in this tracking issue have been addressed.