MNT Make modules private in feature_extraction

[thomasjpfan]

Reference Issues/PRs

Partially addresses #9250

CC @NicolasHug @adrinjalali

[NicolasHug]

We should also deprecate cython files ;)

There are stuff in text that should be private I think, like the VectorizerMixin?

Same for image.extract_patches... Not sure what to do with these.

[glemaitre]

LTGM apart of the comment of @NicolasHug which need to be addressed.

[thomasjpfan]

There are stuff in text that should be private I think, like the VectorizerMixin? Same for image.extract_patches... Not sure what to do with these.

Two options:

1. Move to _text and _image and deprecate them.
2. Move to _text and _image and remove them. (They are not documented)

[adrinjalali]

I would move them to _* and deprecate the public ones (through text and image), and not include the undocumented ones in __init__.

[thomasjpfan]

Updated this PR with a simplier approach. I didn't create another file and deprecated VectorizerMixin and extract_patches in the same file.

[thomasjpfan]

CC @NicolasHug

[NicolasHug]

Thanks @thomasjpfan !