nesting with-imported-modules

[Ricardo Wurmus]

Hi Guix,

it seems that it is impossible to nest with-imported-modules.  A gexp
that is wrapped in multiple layers of with-imported-modules won’t depend
on the list of all mentioned modules but only on the outermost.

This is because with-imported-modules sets the current-imported-modules
parameter without checking if the parameter already has a value.

Should nesting be supported?  It seems useful.

--
Ricardo