[Bindings] Refactor Sofa.Component bindings layout - #653

The bindings of SOFA components (bindings/Modules, bindings/SofaExporter) are
moved to bindings/Component, mirroring SOFA's Sofa/Component layout. Each SOFA
module Sofa.Component.<Area>.<Sub> gets a python module of the same name:

  Sofa.SofaBaseTopology     -> Sofa.Component.Topology.Container.Grid
  Sofa.SofaDeformable       -> Sofa.Component.SolidMechanics.Spring
                               Sofa.Component.SolidMechanics.FEM.Elastic
  Sofa.SofaLinearSolver     -> Sofa.Component.LinearSolver.Iterative
  Sofa.SofaLinearSystem     -> Sofa.Component.LinearSystem
  Sofa.SofaConstraintSolver -> Sofa.Component.Constraint.Lagrangian.Solver
  SofaExporter              -> Sofa.Component.IO.Mesh

Sofa.FEM and Sofa.GL are not components: their bindings (previously
Sofa.SofaFEM and Sofa.SofaGL) move next to Sofa.Core in bindings/Sofa.

- A binding is only built when the SOFA module it binds is available.
- The old python modules are kept as deprecated aliases re-exporting the new
  ones, and Bindings.Modules as a cmake alias of Bindings.Component. Outdated
  compiled modules are removed from the build tree so they do not shadow them.
- SP3_add_python_module: compute the macOS rpath to site-packages from the
  module depth, and set INSTALL_NAME_DIR on the module target.
- Stub generation: for SOFA modules having a compiled binding, the stubs of
  the components without binding are merged in a .pyi next to the binding
  instead of a package that would shadow it.
- Tests, examples and docs use the new module names. The matrix access test
  scenes now load the plugins they use instead of relying on test order.

Component bindings are now compiled modules (e.g. `Sofa/Component/SolidMechanics/Spring.<ext>`), so the stub packages written by `sofaStubgen.py` (`.../Spring/__init__.py`) would have shadowed them.

- **Bound SOFA modules get a `.pyi` next to the binding:** the pybind11/mypy stub of the binding, plus placeholder classes for that module's components that have no binding. Classes that are already bound are skipped. Modules without bindings keep the previous package layout.
- `generate_module_stubs()` now handles single-module output, and a warning is printed when an existing directory would shadow a binding.
- **Generated stubs were mostly invalid on master (309 of 362 did not parse), now fixed:**
  - constructor arguments are now keyword-only and comma-separated;
  - keyword, duplicate and invalid data field names are filtered out;
  - docstrings are escaped;
  - `TypeHints.SofaArray` was replaced by `SofaArray`, which is the name actually imported.
- **Found by the full pybind11-stubgen run:**
  - `LinearSpring`'s `restLength` now defaults to `0` (as in C++);
  - `from __future__` imports are moved to the top of generated `.pyi` files.
- **Behavior change:** for the 7 bound modules, placeholders of unbound components exist only for type checkers and can no longer be imported at runtime.
- **Tested:** all 369 generated stubs compile and execute