CMake: Quick, Local Dependencies (#1699)

This adds new options to CMake for developers that like to build
against local dependencies. For example:

```
cmake -S . -B build -DWarpX_amrex_src=../amrex
cmake --build build -j 8
```

brings back the full "build my local source, I am working on it"-
experience.

The *Python* equivalent looks like this:

```bash
WarpX_amrex_src=$PWD/../amrex python3 -m pip install -v .
```
(`pip` runs in a temporary directory, so path hints need to be absolute. All details in the docs.)

Added for all auto-downloaded dependencies:
- AMReX
- openPMD-api
- PICSAR

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