Flatgfa filesystem restructure

[susan-garry]

The flatgfa filesystem now contains two submodules:

• fgfa_ds, which contains the definitions for the flatgfa datastructure
• commands, which contains our implementations of the odgi commands using flatgfa

After experimenting with sub-packages, I decided that it was simpler to use submodules for now (and until/if they become infeasible) owing to the fact that we would like to expose details in fgfa_ds to flatgfa-py, but the rust compiler ignores sub-packages by default and I couldn't find a workaround for this.