Partly fixes <https://issues.guix.gnu.org/49439>.
Reported by Ricardo Wurmus <rekado@elephly.net>.
When running the command:
guix environment pigx-scrnaseq --search-paths --no-grafts
this change reduces total heap allocations from 1.4GiB to 717MiB (49%)
and wall-clock time from 7.5s to 5.7s (24%).
Without '--no-grafts', heap allocations go from 2.1GiB to 1.4GiB (33%)
and wall-clock time from 12.1s to 10.9s (10%).
* guix/derivations.scm (coalesce-duplicate-inputs): Rewrite using a hash
table to make it O(N) rather than O(N²).
