Re-organize Skia recipes

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

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