No functional changes.
Moving the following functions
`action_fcurve_ensure` and
`action_fcurve_find`
from
`ANIM_fcurve.hh`/`fcurve.cc`
to
`ANIM_action.hh`/`action.cc`
This is an effort to ensure that the fcurve files don't need to know
about the container they are stored in so we can swap out
the container more easily.
Pull Request: https://projects.blender.org/blender/blender/pulls/114575