* guix/search-paths.scm (search-path-definition): New procedure.
* guix/scripts/environment.scm (for-each-search-path): Rename to...
(evaluate-input-search-paths): ... this. Remove 'proc' and 'pure?'
parameters, and return directly the list of search-path/value pairs.
(create-environment): Use 'for-each' and 'evaluate-input-search-paths'
instead of 'for-each-search-path'.
(show-search-paths): Use 'for-each', 'search-path-definition', and
'evaluate-search-paths' instead of 'for-each-search-path'.