go rework

This commit includes:
* a new go package that uses gccgo to bootstrap the go toolchain
* env support added to Executable
* a new Go fetch strategy that uses `go get` to fetch a package and all
  of its deps
* A platinum searcher package that leverages the new go package and
  fetch strategy

var/spack/repos/builtin/packages/the_platinum_searcher/package.py

@@ -1,19 +1,20 @@
 from spack import *
+import os
+import shutil
+
 
 class ThePlatinumSearcher(Package):
     """Fast parallel recursive grep alternative"""
     # FIXME: add a proper url for your package's homepage here.
     homepage = "https://github.com/monochromegane/the_platinum_searcher"
-    url      = "https://github.com/monochromegane/the_platinum_searcher/archive/v1.7.7.tar.gz"
+    url      = "https://github.com/monochromegane/the_platinum_searcher"
 
-    version('1.7.7', '08d7265e101bc1427d5d4b9903aa1166')
+    package = 'github.com/monochromegane/the_platinum_searcher/...'
 
-    depends_on("go")
+    version('head', go=package)
+
+    extends("go")
 
     def install(self, spec, prefix):
-        env = which('env')
-        env()
-        # Fetch all dependencies
-        go('get', './...')
-        # Build pt
-        go('build')
+        go('install', self.package)
+        shutil.copytree('bin', os.path.join(prefix, 'bin'))