Run single presubmit steps.
Add ability to run single presubmit steps instead of always
running everything.
Many small fixes:
* add CIPD lib directories to LD_LIBRARY_PATH
* set PW_ROOT (pretty sure not needed but matches user workflows)
* set VIRTUAL_ENV
* add "--editable=" to pip install (it's inexplicably required)
* search for Pigweed Python packages without Pigweed environment
(the virtualenv seems to screw with vpython)
* install from Pigweed requirements.txt
Bug: 3, 29
Change-Id: I549d7820015f79d315de43e7cfca4c692ace1c96
5 files changed