guix build: Add ‘--dependents’.

* guix/scripts/build.scm (show-help, %options): Add ‘--dependents’.
(dependents): New procedure.
(options->things-to-build): Add ‘store’ parameter; honor ‘dependents’
option.
[for-type]: Handle ‘dependents’ type.
(options->derivations): Update call to ‘options->things-to-build’.
* tests/guix-build.sh: Add test.
* doc/guix.texi (Additional Build Options): Document ‘--dependents’.
(Invoking guix refresh): Cross-reference it.
* doc/contributing.texi (Submitting Patches): Mention it.

Change-Id: I00b6d5831e1f1d35dc8b84a82605391d5a8f417c

tests/guix-build.sh

@@ -196,6 +196,12 @@ test `guix build -D hello -d \
       | grep -e 'glibc.*\.drv$' -e 'gcc.*\.drv$' -e 'binutils.*\.drv$' \
       | wc -l` -ge 3
 
+# Building the dependents.
+test `guix build -P1 libgit2 -P1 libssh -d \
+      | grep -e 'guile-git.*\.drv$' -e 'guile-ssh.*\.drv$' \
+             -e 'libgit2.*\.drv$' -e 'libssh.*\.drv$' \
+      | wc -l` -eq 4
+
 # Unbound variable in thunked field.
 cat > "$module_dir/foo.scm" <<EOF
 (define-module (foo)