Summary:
Pull Request resolved: https://github.com/facebookexperimental/eden/pull/109

Bring back getdeps build of Mononoke so it can be built in opensource.

Includes cargo generation and marking a couple of comments as text to stop cargo trying to build them as rust doc tests

Successful GitHub Linux Build: https://github.com/facebookexperimental/eden/runs/5147129282?check_suite_focus=true

Successful GitHub Mac Build:
https://github.com/facebookexperimental/eden/runs/5147129271?check_suite_focus=true

Reviewed By: markbt

Differential Revision: D34114848

fbshipit-source-id: a18f4370048054b087d421deced29f0b7ba7ada1