build-system/go: Add pre-built standard library as implicit input.

* gnu/packages/golang.scm (make-go-std): New procedure.
* guix/build-system/go.scm (make-go-std): New procedure.
(lower): Use it.  Add pre-built standard library to inputs.

Signed-off-by: Leo Famulari <leo@famulari.name>

gnu/packages/golang.scm

@@ -831,6 +831,36 @@ in the style of communicating sequential processes (@dfn{CSP}).")
 
 (define-public go go-1.14)
 
+(define-public (make-go-std go)
+  "Return a package which builds the standard library for Go compiler GO."
+  (package
+    (name (string-append (package-name go) "-std"))
+    (version (package-version go))
+    (source #f)
+    (build-system go-build-system)
+    (arguments
+     `(#:import-path "std"
+       #:build-flags `("-pkgdir" "pkg") ; "Install" to build directory.
+       #:allow-go-reference? #t
+       #:substitutable? #f ; Faster to build than download.
+       #:tests? #f ; Already tested in the main Go build.
+       #:go ,go
+       #:phases
+       (modify-phases %standard-phases
+         (delete 'unpack)
+         (replace 'install
+           (lambda* (#:key outputs #:allow-other-keys)
+             (let* ((out (assoc-ref outputs "out"))
+                    (out-cache (string-append out "/var/cache/go/build")))
+               (copy-recursively (getenv "GOCACHE") out-cache)
+               (delete-file (string-append out-cache "/trim.txt"))
+               (delete-file (string-append out-cache "/README")))))
+         (delete 'install-license-files))))
+    (home-page (package-home-page go))
+    (synopsis "Cached standard library build for Go")
+    (description (package-description go))
+    (license (package-license go))))
+
 (define-public go-0xacab-org-leap-shapeshifter
   (let ((commit "0aa6226582efb8e563540ec1d3c5cfcd19200474")
         (revision "12"))