Merge remote-tracking branch 'aosp/upstream'

* aosp/upstream:
  WalkDeps - only record module visited when it has been recursed into
  Allow missing variants when allowMissingDependencies=true
  bpmodify: fix os.Exit() shouldn't shadow panic()
  bpmodify: respect exitCode
  Set bpmodify usage function

Test: treehugger
Change-Id: Ida99d7412ea3c034e1ddce228b1e851c1dc40cc2