[roll] Roll infra/recipes [checkout] Rearrange methods

1. Reorder methods to be roughly in call order; e.g. from_spec ->
   fuchsia_with_options -> with_options. This makes the code easier to
   read.
2. Move `project()` up with the other public methods to conform to the
   recipe style guide: http://go/fuchsia-recipe-docs#function-ordering
3. Inline `finalize_checkout()` into `fuchsia_with_options()` since it's
   only called there. `finalize_checkout()` doesn't make it very clear
   what the function does, so inlining the code makes
   `fuchsia_with_options()` easier to understand.
4. Make the `_from_patchset()` and `_from_commit()` methods private.
   They're only called from this module and are not particularly useful
   for other recipes, as they provide fairly low-level functionality.

This is a pure refactoring, so no expectation file diffs.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/recipes/+/516525
Original-Revision: 6a8e841b4c95b414f53ef11f9db05c2b9bdf7d42
GitOrigin-RevId: 9cecfc1c65790a69bcc88b6367943e2600e4338e
Change-Id: Ic82d0c4390c8e6614f18b4fcb2c97b3346a86653