Rename <package-source> to <origin>; use the `letrec*' behavior in packages.

* guix/packages.scm (<package-source>): Rename to...
  (<origin>): ... this.  Update users.

* distro/base.scm (libsigsegv, gawk, hello): Adjust to renaming; refer
  to VERSION to build the URL.

distro/base.scm

@@ -32,9 +32,11 @@
   (package
    (name "libsigsegv")
    (version "2.10")
-   (source (source
+   (source (origin
             (method http-fetch)
-            (uri "http://ftp.gnu.org/gnu/libsigsegv/libsigsegv-2.10.tar.gz")
+            (uri (string-append
+                  "http://ftp.gnu.org/gnu/libsigsegv/libsigsegv-"
+                  version ".tar.gz"))
             (sha256
              (base32 "16hrs8k3nmc7a8jam5j1fpspd6sdpkamskvsdpcw6m29vnis8q44"))))
    (build-system gnu-build-system)
@@ -54,9 +56,10 @@ handlers, distributed shared memory, and more.")
   (package
    (name "gawk")
    (version "4.0.0")
-   (source (source
+   (source (origin
             (method http-fetch)
-            (uri "http://ftp.gnu.org/gnu/gawk/gawk-4.0.0.tar.bz2")
+            (uri (string-append "http://ftp.gnu.org/gnu/gawk/gawk-" version
+                                ".tar.bz2"))
             (sha256
              (base32 "0sss7rhpvizi2a88h6giv0i7w5h07s2fxkw3s6n1hqvcnhrfgbb0"))))
    (build-system gnu-build-system)
@@ -88,9 +91,10 @@ code.")
   (package
    (name "hello")
    (version "2.8")
-   (source (source
+   (source (origin
             (method http-fetch)
-            (uri "http://ftp.gnu.org/gnu/hello/hello-2.8.tar.gz")
+            (uri (string-append "http://ftp.gnu.org/gnu/hello/hello-" version
+                                ".tar.gz"))
             (sha256
              (base32 "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))))
    (build-system gnu-build-system)

guix/packages.scm

@@ -29,12 +29,12 @@
             location-line
             location-column
 
-            source
-            package-source?
-            package-source-uri
-            package-source-method
-            package-source-sha256
-            package-source-file-name
+            origin
+            origin?
+            origin-uri
+            origin-method
+            origin-sha256
+            origin-file-name
             base32
 
             package
@@ -93,15 +93,15 @@ etc."
     (location file (and line (+ line 1)) col)))
 
 
-;; The source of a package, such as a tarball URL and fetcher.
-(define-record-type* <package-source>
-  source make-package-source
-  package-source?
-  (uri       package-source-uri)                     ; string
-  (method    package-source-method)                  ; symbol
-  (sha256    package-source-sha256)                  ; bytevector
-  (file-name package-source-file-name                ; optional file name
-             (default #f)))
+;; The source of a package, such as a tarball URL and fetcher---called
+;; "origin" to avoid name clash with `package-source', `source', etc.
+(define-record-type* <origin>
+  origin make-origin
+  origin?
+  (uri       origin-uri)                          ; string
+  (method    origin-method)                       ; symbol
+  (sha256    origin-sha256)                       ; bytevector
+  (file-name origin-file-name (default #f)))      ; optional file name
 
 (define-syntax base32
   (lambda (s)
@@ -120,7 +120,7 @@ representation."
   package?
   (name   package-name)                   ; string
   (version package-version)               ; string
-  (source package-source)                 ; <package-source> instance
+  (source package-source)                 ; <origin> instance
   (build-system package-build-system)     ; build system
   (arguments package-arguments            ; arguments for the build method
              (default '()))
@@ -155,7 +155,7 @@ representation."
 (define (package-source-derivation store source)
   "Return the derivation path for SOURCE, a package source."
   (match source
-    (($ <package-source> uri method sha256 name)
+    (($ <origin> uri method sha256 name)
      (method store uri 'sha256 sha256 name))))
 
 (define* (package-derivation store package