profiles: Move some of the work to the build side.

profiles: Move some of the work to the build side.

When running:

  guix environment --ad-hoc gnome --no-grafts --search-paths

this reduces wall-clock time by ~5%.  The number of object cache lookups
goes down from 96K to 89K.  (Note that 'gnome' is an interesting example
because it has many propagated inputs, which themselves have propagated
inputs too, which would lead to a long input list and a long manifest in
the 'profile-derivation' gexp.)

* guix/profiles.scm (profile-derivation)[inputs, search-paths]: Remove.
[extra-inputs]: New variable.
[builder]: Adjust call to 'build-profile'.
* guix/build/profiles.scm (manifest-sexp->inputs+search-paths): New
procedure.
(build-profile): Remove 'inputs' parameter; make 'manifest' the 2nd
positional parameter and add #:extra-inputs.  Call
'manifest-sexp->inputs+search-paths' to obtain 'inputs' and
'search-paths'.