explicitly hoist Canvas::RootAccountCacher out of module
explicitly hoist Canvas::RootAccountCacher out of module

closes FOO-2449
flag=none

For zeitwerk this appears to be a problem because of
the other reference to a "Canvas" constant inside
the class in the STI area which seems to stop
the loader from considering it a module elsewhere

TEST PLAN:
  1) specs still pass

Change-Id: If32f97c9b205c25c6a61cc7dcb23defe1151e907
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275416
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>
Product-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms