Break Skia recipe module into:
- skia_vars: defines and stores variables (eg. paths)
- skia_step: utilities for running Skia steps
- skia_flavor: flavor-specific stuff
- skia: top-level setup, checkout, test/perf steps, etc etc
This establishes a saner dependency structure for the recipes; skia_vars
is at the bottom level, skia_step depends on it, skia_flavor depends on
both of them, skia depends on all of the above, and the recipes
themselves may depend on any or all of them.
Next steps:
- Merge buildbot_spec into skia_vars
- Move test_steps and perf_steps from skia recipe_module into
swarm_test and swarm_perf recipes
- Cleaner checkout_steps process
BUG=skia:5578
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2198173002
Review-Url: https://codereview.chromium.org/2198173002
