Extend channel-build-system?

[Nicolas Graves]

Hi Guix,

I am struggling to build a channel with properly defined metadata and/or
proper dependencies.  This is because the current channel-builds-system
is only able to build a guix channel, rather than any channel.

Also, the error reporting is not great when trying to build another channel:
```
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure struct-vtable: Wrong type argument in position 1 (expecting 
struct): #f
```

IIUC, this is because in `channel-instance-derivations` needs a
`core-instance`.  In the context of another channel than guix, it could
be extracted either from channel metadata dependencies or from inputs.

WDYT?  Do you already know of a way to do this?

-- 
Best regards,
Nicolas Graves