replace import nesting tests with tach

[drammock]

This PR uses tach as a pre-commit hook to test submodule interdependence. TODO:

• [x] make sure it runs in pre-commmit.ci (https://github.com/mne-tools/mne-python/actions/runs/13843290676/job/38735774113?pr=13160#step:4:153)
• [x] change the tach.toml config to stop allowing an existing internal dependency, and make sure the CI fails. (https://github.com/mne-tools/mne-python/actions/runs/13843379560/job/38736078349?pr=13160#step:4:116)
• [x] change the codebase to actually remove that internal dependency, and make sure CI passes. https://github.com/mne-tools/mne-python/actions/runs/13843411801/job/38736168047?pr=13160#step:4:116
• [ ] go through tests/test_import_nesting.py and make sure tach.toml reflects all the expectations we've set there. When it does, we can remove those tests.
• [ ] add a visualization of the dependency graph to our docs somewhere? (it can output .dot files)