Move switch-symlinks to (guix build utils)

[Arun Isaac]

Hi Guix,

The switch-symlinks function from (guix utils) is often required in
activation-service G-expressions. But, only (guix build utils) and not
(guix utils) is available to such G-expressions. See, for example, the
pcscd-activation G-expression in (gnu services security-token) where
switch-symlinks gets
redefined. 
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/services/security-token.scm#n70

Could we move switch-symlinks to (guix build utils)? Would such a change
require a core-updates cycle?

Thanks!
Arun