utils: Add 'search-input-directory'.

* guix/build/utils.scm (search-input-directory): New procedure.
* doc/guix.texi (Build Utilities): Document it next to
'search-input-file'.  Tweak wording.

doc/guix.texi

@@ -8677,10 +8677,15 @@ Return the complete file name for @var{program} as found in
 @end deffn
 
 @deffn {Scheme Procedure} search-input-file @var{inputs} @var{name}
-Return the complete file name for @var{name} as found in @var{inputs}.
-If @var{name} could not be found, an exception is raised instead.
-Here, @var{inputs} is an association list like @var{inputs} and
-@var{native-inputs} as available to build phases.
+@deffnx {Scheme Procedure} search-input-directory @var{inputs} @var{name}
+Return the complete file name for @var{name} as found in @var{inputs};
+@code{search-input-file} searches for a regular file and
+@code{search-input-directory} searches for a directory.  If @var{name}
+could not be found, an exception is raised.
+
+Here, @var{inputs} must be an association list like @code{inputs} and
+@code{native-inputs} as available to build phases (@pxref{Build
+Phases}).
 @end deffn
 
 Here is a (simplified) example of how @code{search-input-file} is used

guix/build/utils.scm

@@ -81,6 +81,7 @@ (define-module (guix build utils)
             list->search-path-as-string
             which
             search-input-file
+            search-input-directory
             search-error?
             search-error-path
             search-error-file
@@ -633,6 +634,22 @@ (define (search-input-file inputs file)
      (or (search-path directories file)
          (raise (condition (&search-error (path directories) (file file))))))))
 
+(define (search-input-directory inputs directory)
+  "Find a sub-directory named DIRECTORY among the INPUTS and return its
+absolute file name.
+
+DIRECTORY must be a string like \"xml/dtd/docbook\".  If DIRECTORY is not
+found, an exception is raised."
+  (match inputs
+    (((_ . directories) ...)
+     (or (any (lambda (parent)
+                (let ((directory (string-append parent "/" directory)))
+                  (and (directory-exists? directory)
+                       directory)))
+              directories)
+         (raise (condition
+                 (&search-error (path directories) (file directory))))))))
+
 
 ;;;
 ;;; Phases.