The objective is to divide the functionalities of the current SDC plugin into 2 optionally 3 separate plugins:

1. 'sdc' -- The part which reads SDC into the Yosys attributes, i.e. read_sdc and write_sdc
2. 'propagate-constraints' -- The part which propagates the constraints in the Yosys attributes across things like clock buffers and similar, i.e. propagate_clocks (and maybe future things like generate_pll_config or something?)
3. (optionally) 'pcf' -- In case we end up wanting a read_pcf or read_other_timing_constraint_format plugins which pull data into the same Yosys attributes.

In the current SDC plugin implementation the clock information is not assigned to parameters but kept in a separate Clocks class object. The object would have to be global in order to work for all timing related plugins or the clock information would have to be stored in the RTLIL::Wire parameters in form of extra parameters.