Refactor cmake: auto-download and default-on dependencies (#6355)

Refactor cmake: auto-download and default-on dependencies (#6355)

This PR changes a few things in how dependencies are handled during configuration:

- --x-dir are removed for most dependencies, use the generic --dep-path instead
- the cmake ENABLE_AUTO_DOWNLOAD determines whether we attempt to download missing dependencies ourselves
- external projects check this option and send an error if it is OFF
- some optional dependencies are enabled by default (CaDiCaL, Poly, SymFPU)

This will essentially fail every call to ./configure.sh until the user specifies --auto-download.