[RFC] [Feature] In project `configuration.scm` allow dependencies to other repos with `configuration.scm`

[Josh Marshall <joshua.r.marshall.1991@gmail.com>]

Hello all,

I have come across a use case that I don't find well addressed.  If I
am not wrong (and I very well might be) it would be a feature which
could be of great use.  First sent to a Nix channel, but I think the
same applies here.

```
I want to use git repos with Nix Flakes in them directly as if they
were package dependencies in another Flake. I can't seem to find good
documentation for this. Is it even a thing? The purpose is to improve,
but not impose perfection or slowdown, on developers or researchers as
they Nix-ify their projects. The application is that I have a root
project that needs to be put into a flake, which in turn has a number
of other projects which are not Nix packages but could feasibly add a
Nix Flake. The end goal is to be able to set up and package a project
such that the end result is as easy to replicate for others as
described here:
https://obscuratur-in-arena.readthedocs.io/en/latest/A-good-way-to-set-up-your-python-project-using-Nix-Direnv-and-Poetry.html
```

I regret that I must again be someone who just throws more work
demands into the ether.  The whole "I am finite" thing.