environment: Add only the specified outputs of the dependencies.

environment: Add only the specified outputs of the dependencies.

Before that, 'guix environment guile' (for instance) would define environment
variables that would refer to the "include" output of Bash, the "debug" output
of libgc, etc., even though these are not listed as inputs in the recipe of
'guile'.

* guix/gexp.scm (lower-inputs): Export.
* guix/scripts/environment.scm (evaluate-input-search-paths): Remove
  'derivations' parameter; add 'search-paths'.  Expect 'inputs' to be
  a list of tuples.  Adjust callers.
  (create-environment): Remove 'derivations' parameter; add 'search-paths'.
  (show-search-paths): Likewise.
  (package+propagated-inputs): New procedure.
  (packages->transitive-inputs, packages+propagated-inputs): Remove.
  (build-inputs): Expect INPUTS to be a list of derivation tuples.
  (guix-environment): Compute INPUTS using 'package+propagated-inputs',
  'package->bag', and 'bag-transitive-inputs'.  Move 'run-with-store' higher.
* tests/guix-environment.sh: Add test with FINDUTILS-BOOT0.