Resolved by applying this advice (which is output when running
`guix package -i pango` explicitly, but not when pango is
installed as a dependency -- or I missed it):
```
The following environment variable definitions may be needed:
export
XDG_DATA_DIRS="$HOME/.guix-profile/share${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"
export
GIO_EXTRA_MODULES="$HOME/.guix-profile/lib/gio/modules${GIO_EXTRA_MODULES:+:}$GIO_EXTRA_MODULES"
```
(Besides, the web site gives other mailing lists (not in the info
manual)... maybe help-guix is better suited for this kind of
issues.)
