profiles: Add 'package->development-manifest'.

* guix/profiles.scm (package->development-manifest): New procedure.
* guix/scripts/environment.scm (input->manifest-entry)
(package-environment-inputs): Remove.
* guix/scripts/environment.scm (options/resolve-packages): Use
'package->development-manifest' instead of 'package-environment-inputs'.
* tests/profiles.scm ("package->development-manifest"): New test.

guix/profiles.scm

@@ -124,6 +124,7 @@
 
             profile-manifest
             package->manifest-entry
+            package->development-manifest
             packages->manifest
             ca-certificate-bundle
             %default-profile-hooks
@@ -400,6 +401,24 @@ file name."
                      (properties properties))))
     entry))
 
+(define* (package->development-manifest package
+                                        #:optional
+                                        (system (%current-system))
+                                        #:key target)
+  "Return a manifest for the \"development inputs\" of PACKAGE for SYSTEM,
+optionally when cross-compiling to TARGET.  Development inputs include both
+explicit and implicit inputs of PACKAGE."
+  (manifest
+   (filter-map (match-lambda
+                 ((label (? package? package))
+                  (package->manifest-entry package))
+                 ((label (? package? package) output)
+                  (package->manifest-entry package output))
+                 ;; TODO: Support <inferior-package>.
+                 (_
+                  #f))
+               (package-development-inputs package system #:target target))))
+
 (define (packages->manifest packages)
   "Return a list of manifest entries, one for each item listed in PACKAGES.
 Elements of PACKAGES can be either package objects or package/string tuples