Rationale:
- npm and Yarn both put (a directory containing) the eslint binary on PATH during "npm run foo", so take advantage of that to invoke eslint in a way that is compatible with Yarn workspaces.
- (Node that `node_modules/eslint/...` may not exist, because the module may have been installed in another `node_modules` further up the filesystem tree. Yarn does ensure that the binary gets symlinked into `node_modules/.bin`.)
- This should fix issue google#490.