refactor(measure_theory): add subfolder (#8594)
* This PR adds the subfolders `constructions` `function` `group` `integral` and `measure` to `measure_theory`
* File renamings:
```
group -> group.basic
prod_group -> group.prod
bochner_integration -> integral.bochner
integration -> integral.lebesgue
haar_measure -> measure.haar
lebesgue_measure -> measure.lebesgue
hausdorff_measure -> measure.hausdorff
```
