profiles: Fix pathological performance of 'manifest-transitive-entries'.

For packages with lots of propagated inputs,
'manifest-transitive-entries', as called from 'check-for-collisions',
would exhibit pathological behavior.  For example, "guix install cl-ana"
wouldn't complete in 1mn; now, it's down to 20s.

The issue was that manifest entries would never be 'equal?' due to the
delayed field in <manifest-entry>.

* guix/profiles.scm (manifest-transitive-entries): Use a vhash instead
of a set.  Use 'manifest-entry=?' instead of 'equal?' when checking for
equality.