guix: Unify fetch methods
guix: Drop unneeded modules
guix: Specify symbols in modules explicitly
This change improves the maintainability of the manifest:
(1) It allows to remove the module when the specified symbols are no longer used.
(2) It prevents accidental use of other symbols, such as bash instead of bash-minimal.
This is a backport of core#27811
Depends on D15344