[recipes] Rename some modules and files

- Move doxygen and binary size out of core and into their own modules.
- Rename core -> checkout since that's all it does, shorten method names.
- Rename flavors:
  - Everything is GN, so remove GN/gn_ everywhere.
  - Merge gn_flavor into default.
  - Shorten file / module names.

Bug: skia:6473
Change-Id: I8ac9ff9c9a267f366206b9991adfa5eb37126ca7
Reviewed-on: https://skia-review.googlesource.com/129176
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
49 files changed