Refactor `eko.io.runcards.Legacy`

[felixhekhorn]

Left over from #417

then continuing from #175:

• we still have eko.io.runcards.Legacy: 1. we should rename that to NnpdfDb or similar (and maybe move it?) and 2. I guess we need to maintain that also midterm (or even forever)

[felixhekhorn]

This also caused confusion here NNPDF/pineko#205