base: Pass the correct load path to the 'evaluate' command.

The previous load path was potentially incorrect since commit
2fe7ff87e2.

* src/cuirass/base.scm (evaluate)[tokenize, load-path]: New variables.
Assume #:load-path is colon-separated.  Pass LOAD-PATH as the second
argument to 'evaluate'.
* doc/cuirass.texi (Database): Adjust documentation.

doc/cuirass.texi

@@ -11,7 +11,8 @@ This manual is for Cuirass version @value{VERSION}, a build automation
 server.
 
 Copyright @copyright{} 2016, 2017 Mathieu Lirzin@*
-Copyright @copyright{} 2017 Mathieu Othacehe
+Copyright @copyright{} 2017 Mathieu Othacehe@*
+Copyright @copyright{} 2018 Ludovic Courtès
 
 @quotation
 Permission is granted to copy, distribute and/or modify this document
@@ -248,12 +249,16 @@ specification itself.
 The URL of the repository.
 
 @item load_path
-This text field holds the name of the subdirectory in the checked out
-repository that is passed to the @code{evaluate} tool as the Guile load
-path.  This directory is interpreted relative to the repository in the
-Cuirass cache directory.  This will usually be the current directory
+This field holds a colon-separated list of directories that are
+prepended to the Guile load path when evaluating @code{file} (see
+below.)
+
+Each entry that is not an absolute file name is interpreted relative to
+the source code checkout.  Often, @code{load_path} has just one entry,
 @code{"."}.
 
+When @code{load_path} is empty, the load path is left unchanged.
+
 @item file
 The absolute name of the Scheme file containing PROC.
 

src/cuirass/base.scm

@@ -253,12 +253,24 @@ in SOURCE directory.  Return a list of jobs."
         (#:system . ,(derivation-system drv))
         ,@job)))
 
+  (define (tokenize str)
+    (string-tokenize str (char-set-complement (char-set #\:))))
+
+  (define load-path
+    (match (assq-ref spec #:load-path)
+      (#f
+       "")
+      ((= tokenize path)
+       (string-join (map (lambda (entry)
+                           (if (string-prefix? "/" entry)
+                               entry
+                               (string-append source "/" entry)))
+                         path)
+                    ":"))))
+
   (let* ((port (non-blocking-port
-                (open-pipe* OPEN_READ
-                            "evaluate"
-                            (string-append (%package-cachedir) "/"
-                                           (assq-ref spec #:name) "/"
-                                           (assq-ref spec #:load-path))
+                (open-pipe* OPEN_READ "evaluate"
+                            load-path
                             (%guix-package-path)
                             source (object->string spec))))
          (result (match (read/non-blocking port)