bug#38529: Make --pure the default for `guix environment'?

[raingloom]

On Tue, 2019-12-10 at 18:16 +0100, Ludovic Courtès wrote:
> As for ‘--ad-hoc’: making it the default is technically easy.  The
> difficulty is to come up with a nice transition/deprecation mechanism
> so
> that we don’t break everyone’s script overnight.
> 
> Ideas on how to achieve it are welcome!
> 
> Thanks,
> Ludo’.
> 
> 

Why not make it fully explicit, without either being the default?
It would make script more readable too.

Then the deprecation is as easy as printing a warning when the
invocation relies on the default.

For example `guix environment hello` would become `guix environment --
inputs-of hello`. Not using `--inputs-of` would print something like
"warning: implicit --inputs-of is deprecated".