vg cluster does not accept GBZs

[faithokamoto]

vg cluster accepts a graph via its required --xg-name/-x option. Similarly named options in other commands accept other graph formats such as GBZ. Indeed the gloss for this option is "use this xg index or graph". However, attempting to pass a GBZ to -x results in an error:

[IndexRegistry]: Constructing a greedy path cover GBWT
error[VPKG::load_one]: Correct input type not found while loading xg::XG

I see that the filename passed to -x is given to an IndexRegistry as an XG; the issue arises because we promise the IndexRegistry an XG but give it a GBZ.

Would be nice to be able to pass GBZs to -x and have them auto-detected somehow.

Version:

vg version v1.65.0-2-gd6152a490 "Carfon"
Compiled with g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0 on Linux
Linked against libstd++ 20230528
Using HTSlib headers 101990, library 1.19.1-29-g3cfe8769
Built by fokamoto@mustard