more package transformations: overrides

[Ricardo Wurmus]

Hi Guix,

when using package transformations the command line can become crowded
very quickly.  Sometimes I would like to be able to provide a file
containing a set of alternative packages that are used recursively in
the package dependency graph, as some sort of override.

What do you think about extending package transformations to allow for
reading override packages from a file?  The replacements would happen by
matching package names, so a package named “python-pytorch” in my
overrides file would replace the original “python-pytorch” package in
the input graph.  All these packages would be applied as overrides
together.

-- 
Ricardo