packages: Allow file names as package sources.

* guix/packages.scm (package-source-derivation): Add cases for SOURCE as
  a store path or user file.
* tests/packages.scm ("package-source-derivation, file",
  "package-source-derivation, store path"): New tests.

guix/packages.scm

@@ -250,7 +250,11 @@ corresponds to the arguments expected by `set-path-environment-variable'."
   (match source
     (($ <origin> uri method sha256 name)
      (method store uri 'sha256 sha256 name
-             #:system system))))
+             #:system system))
+    ((and (? string?) (? store-path?) file)
+     file)
+    ((? string? file)
+     (add-to-store store (basename file) #t "sha256" file))))
 
 (define (transitive-inputs inputs)
   (let loop ((inputs  inputs)

tests/packages.scm

@@ -99,7 +99,27 @@
                    ("d" ,d) ("d/x" "something.drv"))
                  (pk 'x (package-transitive-inputs e))))))
 
-(test-skip (if (not %store) 6 0))
+(test-skip (if (not %store) 8 0))
+
+(test-assert "package-source-derivation, file"
+  (let* ((file    (search-path %load-path "guix.scm"))
+         (package (package (inherit (dummy-package "p"))
+                    (source file)))
+         (source  (package-source-derivation %store
+                                             (package-source package))))
+    (and (store-path? source)
+         (valid-path? %store source)
+         (equal? (call-with-input-file source get-bytevector-all)
+                 (call-with-input-file file get-bytevector-all)))))
+
+(test-assert "package-source-derivation, store path"
+  (let* ((file    (add-to-store %store "guix.scm" #t "sha256"
+                                (search-path %load-path "guix.scm")))
+         (package (package (inherit (dummy-package "p"))
+                    (source file)))
+         (source  (package-source-derivation %store
+                                             (package-source package))))
+    (string=? file source)))
 
 (test-assert "return values"
   (let-values (((drv-path drv)