cmake: BUILD_DEPS does not include a BUILD_SWIG option

**What language and solver does this apply to?**
CMake build

**Describe the problem you are trying to solve.**
I'm building OR-Tools Python package from source on an Ubuntu machine:
```
cmake -S. -Bbuild -DBUILD_DEPS:BOOL=ON -DBUILD_PYTHON=ON -DCMAKE_BUILD_TYPE=Release
```
I don't have the rights to install packages from apt, so I build the dependencies from source.

Everything is ok when I build the C++ library without Python, but when I build the Python package it fails with `Could NOT find SWIG (missing: SWIG_EXECUTABLE SWIG_DIR)`. Apparently, BUILD_DEPS does not include SWIG.

**Describe the solution you'd like**
Fetch and build SWIG from source when both `BUILD_DEPS` and `BUILD_PYTHON` are on.