[MRG, MAINT] Move freesurfer functions in mne/source_space.py to mne/_freesurfer.py

[alexrockhill]

Helps with refactoring for #9520.

The freesurfer lookup table access seems oddly placed in source_space.py. I think utils.config.py makes sense because that is where get_subjects_dir is.

This would allow the lookup table to be accessed for plotting ROIs in other modules.

[alexrockhill]

I don't think this should get a whatsnew entry since it's just a util but happy to do so if requested.

[larsoner]

source_space.py is where a lot (most?) of our FreeSurfer mangling code lives. I agree source_space.py is at least a misnomer but we can't rename it for backward compat reasons -- perhaps we should make a new mne/_freesurfer.py or so rather than pack more stuff into utils, especially utils/config.py which should really just be about dealing with user config vars

[alexrockhill]

source_space.py is where a lot (most?) of our FreeSurfer mangling code lives. I agree source_space.py is at least a misnomer but we can't rename it for backward compat reasons -- perhaps we should make a new mne/_freesurfer.py or so rather than pack more stuff into utils, especially utils/config.py which should really just be about dealing with user config vars

You know a lot better about where things go than I do, sounds good as far as naming.

[alexrockhill]

Ok @larsoner, this turned out to take a lot more changes than I thought. Now I get a circular import that get_subjects_dir is everywhere the basic import structure is off. I put an hour of work into it so if you want to advice on how to clean it up/clean it up I think this implements what you're suggesting. Otherwise we can just scale back if this is too much to do right now.

[larsoner]

Let's just scale back, there is also a cost to moving things in terms of tracking blame and changes, and the tangible benefit is also small

[larsoner]

... but now having read the PR's diff it is nice to have all this stuff in one place.

I'll see if I can push a commit to fix the circular import problem

[larsoner]

I think the core problem is get_subjects_dir. That one I think actually makes at least as much sense to have in config.py as _freesurfer.py since it mostly uses get_config then does some value checks and raising (both of which are common in utils/ files) -- I think reverting those changes should fix things.

[agramfort]

there is no public API change here?

[alexrockhill]

there is no public API change here?

The only one would be that mne.utils.get_subjects_dir was moving to mne.get_subjects_dir but Eric put that back because of import issues so everything is exactly the same that is a public function in the API.

[agramfort]

is this an API change @alexrockhill

[alexrockhill]

That looks like it is the API that it was before, I just corrected it because it would have been a change but instead we just imported and noqa'd to shallow reference it in coreg

[agramfort]

@larsoner merge if happy

thx @alexrockhill !

[larsoner]

Thanks @alexrockhill !